(P = 0.003). The mean number of 1,280 APC/spleen for T-treated was 45% greater than that of the U. By contrast, at 500 pg/ml and above, APC were significantly reduced; at 500 pg/ml 27 of 36 suspensions had fewer APC than U (P = 0.002). T-treated suspensions averaged 42% fewer. Comparable studies show that at C of estradiol (E0 of 500 to 5,000 pg/ml and of progesterone (P) of 500 pg to 50 ng/ml, increases in APC occur, but at C of 50 ng/ml and 500 ng/ml of these hormones respectively, APC are reduced. Thus, T, E~, and P enhance at lower C and suppress numbers of APC at higher C. On a molar basis, T is the most potent hormone, with its effects at the lowest C. Findings suggest that serum levels o f T in male infants may suppress, whereas those in female infants may enhance, the proliferation and/or differentiation of antibacterial APC.