The known cytolytic function of the alternative pathway in serum was quantitatively reproduced by combining 11 isolated plasma proteins at their respective serum concentrations. These proteins are: C3, Factor B, Factor D, C3b inactivator, beta1H, native properdin, C5, C6, C7, C8, and C9. In absence of activators of the alternative pathway, this mixture was stable at 37 degrees C as evidenced by lack of consumption of Factor B, C3, and C5. Upon addition of either rabbit erythrocytes or neuraminidase-treated sheep erythrocytes, cell lysis ensued and the extent of lysis was dependent on dose of the component mixture. The dose-response curves obtained with the isolated component mixture and with C4-depleted serum were virtually indistinguishable. Nonactivator erythrocytes (untreated sheep erythrocytes) were not lysed by the component mixture. Deletion of properdin resulted only in a twofold diminution of the hemolytic activity of the component mixture. No immunoglobulin requirement was apparent. These results indicate that the cytolytic systems studied are internally sufficient and capable of coupling the initiation and amplification sequence with the cytolytic membrane attack sequence.