Transplacental human immunodeficiency virus (HIV) antibodies from 11 uninfected and 3 infected infants of HIV-seropositive mothers were measured using end point titration of twofold dilutions of sera by a particle-agglutination method. Four or more sequential serum samples from each infant were collected between birth and age > or = 14 months and analyzed. Linear regression of log2 antibody titers of uninfected infants plotted against age (days) showed an exponential decay in maternal HIV antibodies. The mean half-life of passive HIV antibodies was 23.1 +/- SD 4.2 days (range, 18.2-29.3; 95% confidence interval, 20.2-26.0). The median age of clearance of maternal HIV antibodies was 13.3 months (range, 10.4-15.6). A model assuming first-order exponential decay and production of HIV antibodies was fitted to the time course of antibody titers of the infected infants. Minimal titers in each infected infant occurred at 5.5, 4.9, and 4.0 months, and doubling times of HIV antibodies from that point ranged from 16.8 to 31.4 days.