By a quantitative Western immunoblot analysis, the intracellular levels of two principal sigma subunits, sigma 70 (sigma D, the rpoD gene product) and sigma 38 (sigma S, the rpoS gene product), and of two minor sigma subunits, sigma 54 (sigma N, the rpoN gene product) and sigma 28 (sigma F, the rpoF gene product), were determined in two Escherichia coli strains, W3110 and MC4100. The results indicated that the levels of sigma 54 and sigma 28 are maintained at 10 and 50%, respectively, of the level of sigma 70 in both strains growing at both exponential and stationary phases, but in agreement with the previous measurement for strain MC4100 (M. Jishage and A. Ishihama, J. Bacteriol. 177:6832-6835, 1995), the level of sigma 38 was undetectable at the exponential growth phase but increased at 30% of the level of sigma 70 at the stationary phase. Stress-coupled change in the intracellular level was observed for two sigma subunits: (i) the increase in sigma 38 level and the decrease in sigma 28 level upon exposure to heat shock at the exponential phase and (ii) the increase in sigma 38 level under high-osmolality conditions at both the exponential and stationary phases.