C4 functional activity and antigenic levels were determined in H-2-congenic mouse strains. In strains with the H-2w7 haplotype, the C4 hemolytic activity per unit of residual Ss antigenic activity, after depletion of the nonfunctional Slp-positive molecules was 25-33% that found with other H-2 haplotypes. This reduced hemolytic efficiency was not the result of either a more labile C4 molecule or of the presence of inhibitors. Moreover, other strains with comparable antigenic concentrations of Ss (C4) and Slp has three- to fourfold higher levels of C4 hemolytic activity. Based on these data and previously reported structural differences between C4 molecules from the H-2w7 haplotype compared with other standard H-2 haplotypes, the reduced hemolytic efficiency of this molecule is probably secondary to alterations in the structure of its alpha-chain.