Complement consumption by isolated membranes and walls from Group A streptococci and various other gram-positive microbes has been tested. These microbial structures were found to activate the alternate complement pathway. However, unlike endotoxin, inulin, or other plant polysaccharides, activation of complement by our material was found to bypass properdin. The activating factor(s) also differs from cobra venom in its/their requirement for factor D. Preliminary experiments suggest this factor isolated from membranes to be a protein and to have a mol wt greater than 40-60,000 daltons. Our studies have led us to speculate that the phylogenetic role of the alternate complement pathway may be the primordial nonspecific defense system which has retained certain fundamental aspects up to the present time.

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