A flow cytometry–based assay for detection of immunoglobulin (Ig) class and subclass antibodies in human serum or plasma was developed. With use of this procedure, the presence and relative frequency of antibody activity in the Ig classes and subclasses (IgAI, IgA2, IgD, IgE, IgG1, IgG2, IgG3, IgG4, and IgM) to human immunodeficiency virus type 1 (HIV-1) proteins (gp160, gp120, p66, and p24) was determined in serum or plasma from a cohort of 47 HIV-1–infected, pregnant women. Antibody activity in each of the classes and subclasses was found with differences in frequency depending on the Ig class/subclass and the HIV-1 protein. IgG1 antibodies were the most frequently reactive Ig class/subclass to each protein. Intermediate frequencies of reactivity were found in IgA1, IgG2, IgG3, and IgM class and subclasses and antibodies of the IgA2, IgE, and IgG4 class/subclass the least frequently detected. An unexpected finding was the presence of IgD antibodies to HIV-1 proteins in ±50% of the individuals. The distributions of Ig class/subclass antibodies to the different HIV-1 proteins were compared in sera from 14 mothers giving birth to infants who were determined to be HIV-1 infected with sera from 25 individuals whose infants were not infected. Sera from transmitting mothers contained a broader distribution of class and subclass antibodies compared to sera from nontransmitting women. The single most frequent antibody–antigen combination that was found in the transmitting mother was IgG2–gp160. Combinations of IgG1–IgG2, IgD–IgG2 antibodies to gp160 and IgA1–IgE antibodies to p66 were present more frequently in transmitters compared with the same combinations in nontransmitting mothers' sera. Whereas the significance of these findings are not apparent, increased activity of class and subclass antibodies in transmitting mothers suggest the possibility that previously described antibody-mediated enhancement may be operative in HIV-1 mother–infant transmission.