In this study two major antigens of Schistosoma mansoni, the circulating anodic antigen (CAA) and the circulating cathodic antigen (CCA), were localized ultrastructurally in glomeruli of S. mansoni infected mice. These antigens were studied by direct gold labeling in which anti-CAA and anti-CCA monoclonal antibodies were labeled with 5 and 15 nm gold particles, respectively. CAA and CCA were demonstrable in glomeruli at week 3 in the basement membrane and from 5 weeks in moderately electron-dense material of the mesangial matrix. Both antigens were also encountered in fenestrae of the endothelial cells, in filtration slit pores, and on the luminal membranes of the epithelial cells. It appears that CAA and CCA are arrested by the glomerular basement membrane and deposited in the mesangial matrix. CAA was seen in considerably smaller amounts than CCA. This was ascribed to the fact that CAA, but not CCA, is repelled by the negative charge of the capillary walls and the glomerular basement membrane.