Unsensitized normal human erythrocytes (E) were shown to be lysed when incubated with autologous serum in the presence of zymosan (Zy). The hemolysis proceeded slowly with a relatively constant rate for at least 24 h at 37 degrees C. It was shown that the hemolytic reaction is antibody independent and mediated by complement activation through the alternative pathway and that hemolysis is not due to the decay or inactivation of complement regulators present on the E membrane. The mechanism of the phenomenon was studied by use of several kinds of sera genetically deficient in C3, C5, C7 or C9. The reaction was found to be divided into two stages: in the first step, neither E, C5, C7 nor C9 but Zy, serum factors containing C3 and metal ions are necessary, and in the second step, neither C3 nor metal ions but E, C5, C7 and C9 are necessary. Thus, E seem to be lysed by reactive lysis induced by C5 convertase formed on Zy through alternative complement pathway activation.