By using a direct approach we have demonstrated previously that the classical C pathway produces on sheep erythrocyte membranes functional lesions of about 4 nm diameter. We have now investigated the size of the lesions induced on human erythrocyte membranes by activation of the alternative C pathway. Human red cells were treated with 2-aminoethylisothiouronium bromide (AET) to increase their reactivity with C. They were then lysed hypotonically in the presence of radiolabelled marker molecules of different size which were incorporated into the membranes ghosts during resealing. The membranes were finally treated with human serum together with inulin particles to activate the alternative pathway. This resulted in identical % release of 3H-sucrose from membrane ghosts and of hemoglobin from AET-treated intact erythrocytes. No release of 125I-ovalbumin or 125I-BSA was obtained from membrane ghosts. The release of soluble 3H-inulin was slightly slower and that of 125I-RNase markedly slower than that of 3H-sucrose.