Two hundred and eight pregnancy sera were tested for the presence of antibodies specific for lymphocyte sub‐populations by using the isolated B and T lymphocytes from the women's mating partners. This was done by the microlymphocytotoxicity and the indirect immuno‐fluorescence techniques. Five sera (2.5%) reacted exclusively with B lymphocytes and sixty‐three sera (30.2%) reacted with both B and T lymphocytes; none of the sera was specific for T cells. Several sera, reacting with both B and T lymphocytes, were absorbed with platelets and this procedure revealed nine additional antisera specific for B lymphocyte antigens. Specificity studies on a panel of forty‐eight HLA—ABCD typed individuals indicated that most antisera possibly defined new B‐cell antigens. Family studies established that the antigens defined by these antisera were coded for by genes in the Major Histocompatibility Complex.