In previous studies antilaminin antibodies in the sera of immunized monkeys and rats were found to be toxic to cultured rat embryos. In order to extend these studies to humans, head-fold stage rat embryos were cultured for 48 hours on ten different serum samples from individuals with Chagas' disease. All embryos (n = 20) cultured on these sera were found to be abnormal. Using ELISA, Western immunoblot, and indirect immunofluorescence it could be shown that antibodies in these sera reacted with laminin. That these antilaminin antibodies were, at least in part, responsible for the toxicity was indicated 1) by reduced cultured embryo toxicity for six of seven serum samples after pre-absorption with purified laminin, 2) by demonstrating a relationship between the amount of affinity-purified antilaminin IgG added to control serum for culture and the severity of embryo abnormalities seen at the end of culture, and 3) by the sera's failing to react with other basement membrane proteins, including type IV collagen, fibronectin, osteonectin, and heparan sulfate proteoglycan.