Previous studies comparing IgG subclass concentrations in cord and maternal sera have indicated that IgG1 is transported across the placenta to a greater extent than is IgG2. The purpose of our study was to examine the relationship between the transport of IgG1 and IgG2 and the transport of specific antibodies that are relatively restricted to a particular subclass, either IgG1 or IgG2. The concentrations of total serum IgG1 and IgG2 and those of IgG-anti-tetanus toxoid (TT) and anti-group A streptococcal carbohydrate (GAC) were measured in 30 paired maternal and cord sera. Previous studies have shown that anti-TT in adults is predominantly IgG1, whereas anti-GAC is predominantly IgG2. The mean cord/maternal concentration ratios of IgG1 and anti-TT were similar (1.77 +/- 0.56 and 1.93 +/- 0.67, respectively), but differed significantly (P = 0.0001) from those of IgG2 and anti-GAC (0.99 +/- 0.39 and 1.01 +/- 0.45, respectively). We confirmed the difference in cord/maternal concentration ratios of IgG1 and IgG2 antibodies by measuring IgG1 and IgG2 antibodies specific for Haemophilus influenzae type b capsular polysaccharide; the mean cord/maternal concentration ratio of IgG1-anti-Hib PS was significantly higher than that of IgG2-anti-Hib PS (2.23 +/- 0.83 compared with 0.94 +/- 0.49, P = 0.01). These results indicate that placental transport of IgG antibodies is related to their subclass composition.