The equine alternative complement pathway has been partially characterized and compared to the equine classical activation pathway. A dose-dependent lysis of RbRBC was observed with peak lytic values noted within 10 minutes at 37 degrees C when rabbit red blood cells (RbRBC) were used as an alternative pathway activator. Sheep red blood cells (SRBC) sensitized with rabbit hemolysin or partially purified equine IgM antibodies were equally sensitive to lysis. Dilution of the commercial hemolysin by 1/5 reduced lysis from 90% to 38% in the presence of constant cell numbers. Hemolysis of SRBC peaked at 10 minutes and the majority of lysis occurred within 10 minutes. Dilution of equine sera by as little as 1/5 decreased hemolytic activity for SRBC to 21.5% from greater than 90% with undiluted sera. The alternative pathway protein, equine factor B, was tested using RbRBC and monitored by its differential susceptibility to heat treatment at 50 degrees C. This treatment led to almost complete inactivation after a 15-minute incubation. An apparent heat-dependent decay of certain classical pathway components was also observed after 50 degrees C treatment. This sensitivity was indicated by a reduction in the lytic activity for sensitized SRBC. Treatment for 15 minutes at 56 degrees C with either RbRBC or SRBC was sufficient to abolish hemolytic activity in all equine sera tested. Chelation of cations with 0.04 M EDTA blocked expression of alternative and classical pathway activation; however, chelation of Ca++ ions with 10 mM EGTA containing 1 mM Mg++ ions permitted lysis of the RbRBC but not the SRBC. A dose-related Mg++-ion dependence for RbRBC hemolytic activity was observed as the concentration of Mg++ was increased to 1.0 mM. In addition, our results obtained with pre-colostral foal serum strongly suggest that natural antibody to RbRBC was of little importance in the lysis observed with these cells. These results also show that the equine alternative pathway activation may require Ca++ ions. If Ca++ ions are required, the equine alternative pathway is quite different from any other mammalian complement system so far described. Our results suggest that the alternative pathway of activation is of major importance in the equine complement system. Confirmation of this hypothesis requires both purification of the components involved as well as further characterization.