We have previously reported (Nature 1982. 299: 361) that the transplacental transfer of Dipetalonema viteae microfilariae (mf) can induce an antigen‐specific tolerance in rats. Rats thus tolerized have serum factor(s) which block(s) antigen‐specific lymphocyte proliferation. The results of experiments involving fractionation of antisera from tolerant animals indicate that the inhibitory activity for antigen‐specific blastogenesis resides in IgG antibodies. Absorption of IgG (eluted from protein A) with specific filarial antigens reduced the inhibition from 58% to 9% whereas a similar immunosorption of IgG size fraction (obtained by applying to AcA 34 Ultrogel) resulted in a decrease from 72% to 35%. This suggests that IgG size fraction might include factor(s) derived from mf and was partially blocking the blastogenic response. Since the tolerant animals harbor only mf, we have used radiolabeled mf surface antigens for immunoprecipitation by antisera from tolerant animals. Antibodies from tolerant animals have a different specificity for filarial antigens compared to those from immunocompetent and mf‐resistant rats.