The alternative complement pathway is regarded as the evolutionary ancestral complement system. It is therefore of interest to analyze and characterize factor B of the alternative complement pathway in animals at different levels of evolution. We describe a functional assay which is based on the interaction of factor B from various animal species with Cobra Venom Factor, the snake C3b-analog. The signal system in the assay is C3-conversion in factor B-depleted chicken serum. The results show that a functional C3-convertase can be created with factor B from mammals, aves, and amphibians.