The reaction mechanisms of beta1H were studied. The generation of alternative pathway C3 and C5 convertases on the cell surface as well as in the fluid phase was inhibited by beta1H globulin. The cell preparation bearing the C3b site could bind beta1H with little effect on the C3b hemolytic activity. Bound beta1H was dissociated by the action of C3bINA and C3bINA-treated C3b bearing cell did not bind beta1H anymore. Cell-bound beta1H was also dissociated by the action of B (or Bb). From these and other results, the following conclusions were obtained. The C3b site-bearing cell could bind beta1H on the C3c region of C3b molecules facilitating the C3bINA action on C3b, and beta1H shared the same binding site with B (or Bb) inhibiting the generation of the alternative pathway convertases competitively.