1. Chloride (JCl) and bicarbonate (JHCO3) self-exchange flux in fetal human red cells was studied at 0-38 degrees C as 36Cl- and [14C]HCO3- efflux. 2. Both at 0 and 38 degrees C JCl showed a bell-shaped pH dependence with a broad maximum at pH 7-8. JCl was 99.7% inhibited by the binding of 1.1 x 10(6) 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS) molecules per cell membrane. 3. By raising the temperature from 0 to 38 degrees C JCl, at external concentration (C0) = 150 mM, increased about 200 times to 5 x 10(-8) mol/(cm2 s), the half-time of the tracer efflux being 80 ms at 38 degrees C and pH 7.2. Under conditions where Co = (110 mM-Cl- + 25 mM-HCO3-), JCl also increased by about 200 times, while JHCO3 increased only about 100 times, as temperature was raised from 0 to 38 degrees C. 4. The apparent activation energy (EA) of Janion was not constant, but increased gradually with decreasing temperature. Assuming that the change of EA with temperature consists of two components JCl, under both experimental conditions, had average values of EA = 117-120 under and EA = 73-78 kJ/mol above a 'breaking' point at 15-20 degrees C, while JHCO3 showed an EA = 100 below and EA = 48 kJ/mol above 25 degrees C. 5. We conclude that the anion transport protein becomes built into the membrane at a very early stage of life, and that the kinetics of Janion in fetal red cells show characteristics that are similar to those of adult red cells. 6. Our results suggest that the CO2 transport capacity of the fetal blood can be exploited during rest nearly as efficiently as that of adult blood.