AKR/J (H-2kk) mice have higher serum C3 levels than DBA/2J (H-2dd). The F1 hybrids have intermediate levels. Analysis of the progeny of backcrosses at 21 days of age shows that C3 levels in mice of H-2dk type are significantly higher than those with H-2dd type and lower than those with H-2kk type. In addition, mice of H-2kk, H-2dk, and H-2dd types have C3 levels not significantly different from those of AKR/J, AKD2F1, and DBA/2J respectively. These findings demonstrate linkage between a gene controlling C3 levels and the H-2 complex.