Influenza A and B virus reproduction in the allantoic membrane of the intact chicken embryo was studied quantitatively with particle enumeration procedures. Virus particles were enumerated on the basis of two independent properties; capacity to infect and to cause hemagglutination. The infective property of influenza B virus (Lee) was even more unstable than that of influenza A virus (PR8). Inactivation occurred at a constant logarithmic rate which was independent of the concentration of particles and corresponded with first order reaction kinetics. In allantoic fluid at 35 degrees C. either in vitro or in vivo, Lee virus had a half-life for infectivity of only 85 minutes. In contrast, the hemagglutinating property, like that of PR8, was relatively stable and was not appreciably affected by 12 hours at 35 degrees C. On the basis that the number of non-infective particles is equal to the number of hemagglutinating particles minus the number of infective particles and that the number of cells lining the allnatoic membrane is 1.8 x 10(7), the effects of various particle-cell ratios on the reproductive process were analyzed. Adsorption of infective and non-infective Lee particles occurred at the same logarithmic rate, i.e. about 50 per cent in 72 minutes, and the rate was nearly independent of the particle-cell ratio up to a value of 55. The adsorption capacity of an allantoic cell was at least 44 Lee or 89 PR8 particles. The interval before new particles appeared in the allantoic fluid increased as the particle-cell ratio was decreased with both Lee and PR8. At ratios of 0.2 or less, the appearance time for infective particles was nearly identical to that for hemagglutinating particles with both viruses. At ratios of about 1.0, the "latent period" in the allantoic membrane per se was computed to be 150 to 160 minutes for both Lee and PR8. The number of particles, both infective and hemagglutinating, increased at a constant logarithmic rate for 6 hours or more after the adsorptive period. With Lee virus, at a particle-cell ratio of 5 or less, the doubling time was constant and had a value of 43 minutes. The dynamics of the logarithmic increase period suggest that reproduction corresponds to an autocatalytic reaction in which the rate is proportional to the amount of material produced. When the particle-cell ratio was increased to 10 or more, either with infective or non-infective (inactivated at 35 degrees C. or 22 degrees C.) particles, the doubling time increased to 65 minutes. Comparable effects from high ratios were found with PR8. Non-infective particles accumulated at a rapid rate after the interval of constant logarithmic increase regardless of the particle-cell ratio. This accumulation was even more striking with Lee than with PR8 as was expected because of the shorter half-life of the infective property. With both viruses at particle-cell ratios of 4 or more, a large proportion of the particles were non-infective within a few hours after new particles appeared. At particle-cell ratios of 0.2 or less, the maximal yield was relatively constant, i.e., about 900 to 1400 hemagglutinating particles per cell with Lee and 500 to 900 with PR8. However, even with very low ratios, i.e. 0.001 or less, it was not possible to obtain more than about 160 infective particles per cell with either virus regardless of the interval. As was expected, the lower the ratio, the longer was the interval before maximal yields were produced. At ratios of about 10, the maximal yield was reduced by 50 per cent or more with both viruses. Comparable reductions in yield were obtained whether the high particle-cell ratio was due to infective or non-infective (inactivated at 35 degrees C. or 22 degrees C.) particles. These findings indicate that there is a critical particle-cell ratio above which alterations appear in the dynamics of reproduction of influenza viruses. This ratio has a value of approximately 3. The observed alterations in the reproductive process are discussed in relation to the hypothesis that adsorption of 3 or more infective or non-infective particles per cell induces cell damage.