Lysis of Raji cells could be effected by the 11 proteins of the purified cytolytic alternative pathway of complement (PCAP) not including immunoglobulins. Lysis was dose dependent and slow, reaching completion only after 10-20 hr. Cellular uptake of complement proteins occurred early during incubation as evidenced by the binding of radiolabeled properdin and C9. 86Rb release paralleled C9 uptake and was complete in 2 hr. After inhibition of cell metabolism by puromycin the kinetics of cell lysis paralleled C9 uptake and 86Rb release. Raji cells appear to be weak activators of the alternative pathway since properdin and C9 deposition was approximately 10 times slower than their deposition on strong pathway activators. Control of cell-bound C3b by β1H was reduced and intermediate to strong activators and nonactivators. The results indicate that activation of the alternative pathway by Raji cells leads to formation and binding of the membrane attack complex which in turn produces functional membrane lesions. The initial membrane injury does not result in immediate cell death. The time course of cell lysis was 7 times slower than that of formation of the initial lesion.