By using a kinetic assay, we have examined the role of antibody in the lysis of rabbit erythrocytes (RaRBC) through the alternative complement (C) pathway. Sera from some hypogammaglobulinemic (Hγ) humans and all agammaglobulinemic chickens tested had subnormal activity in the assay. Heated normal human or chicken sera, but not heated Hγ sera, restored activity to deficient Hγ serum and initiated hemolysis in the presence of rabbit serum as C source. Absorption of heated normal human serum with RaRBC, but not with sheep erythrocytes or zymosan, removed its ability to reconstitute deficient Hγ serum. Normal hemolytic activity could be restored to Hγ serum with human IgM, IgG, or colostral IgA, with goat anti-RaRBC IgG, or with an eluate from serum-sensitized RaRBC, but not with myeloma IgA. Restoration of hemolytic activity to Hγ serum could be achieved in a dose-dependent fashion with the F(ab′)2 fragment of IgG. These results suggest that antibody exerts a significant rate-limiting effect on alternative pathway activity in the RaRBC lytic system. This raises the possibility that antibody may be required for efficient alternative pathway activity in vivo and that the pyogenic infections that occur in Hγ individuals are due to inefficient activation and fixation of C3 through either the classical or alternative pathway.