An early step in the initiation of the classical C pathway is the proteolytic activation of component C4 by subcomponent C1-s. We have examined the substrate specificity of murine C1-s (mC1-s) by measuring its proteolytic activity on human and murine C4, and on the murine C4 isotype designated sex-limited protein (Slp). The latter substrate was examined because previous studies have demonstrated that Slp is not cleaved by C1-s, and hence Slp has been assumed to be nonfunctional in the C pathways. Those earlier studies used human, not murine, C1-s, however; a recent report has suggested that Slp is an essential component of a novel complement activation pathway and that the previous failure to observe cleavage of Slp is probably the result of a species incompatibility between Slp and the heterologous human C1-s (hC1-s). The present studies do not support this idea, as we found no evidence of cleavage of Slp by homologous murine C1-s even at enzyme concentrations 10-fold higher than that necessary for 50% cleavage of murine C4 (mC4). We did find a species-specific affect in the cleavage of mC4, where mC1-s is about 10-fold more effective than heterologous hC1-s in cleaving mC4, but mC1-s itself does not distinguish between human and murine C4, cleaving both equally well. Hence mC1-s does not exhibit the species specificity previously found for hC1-s, which shows a several hundred-fold preference for homologous human C4 over murine C4.