IgG subclass distribution of antibodies against human chorionic gonadotropin (hCG) and diphtheria toxoid (DT) was determined in the sera of women recruited into Phase II clinical trials of a beta-hCG based immunocontraceptive vaccine. Serum samples from 30 different individuals were analyzed. Anti-hCG antibody response was predominantly of IgG1 subclass, with a mean titer of 537.94 +/- 560 antibody units (AU). The other 3 subclasses showed considerably lower mean levels (IgG2, 16.46 +/- 8.33; IgG3, 3.22 +/- 8.48; IgG4, 56.65 +/- 82.60 AU). A good correlation was observed between the anti-hCG IgG1 antibody titers (r = 0.57, P < 0.01) and the bioneutralization capacity of sera. However, bioneutralization capacity of the sera from subjects capable of inducing IgG4 response was not significantly different from subjects not showing IgG4 antibodies. Similarly, a dominant IgG1 response was observed against diphtheria toxoid (DT) which has been used as one of the carrier proteins in the vaccine.