To investigate the effects of antigen cross-linking on ELISA results, we assayed antibodies to several purified antigens, using microtitration trays coated with antigens in their native and cross-linked forms. Cross-linking was achieved by a carbodiimide promoted condensation reaction. For several antigens an increase in sensitivity of the ELISA was obtained after cross-linking. The results of a double antibody sandwich assay using the same cross-linked antigens, showed that the affinity for antibodies remained unchanged after limited cross-linking. Therefore, the higher sensitivity of the ELISA is most likely due to a higher affinity of the cross-linked antigens for the plastic carrier. Cross-linking of proteins in a complex antigen preparation obtained from Schistosoma mansoni led to a large increase in sensitivity.