A/J and C57 Br/cdj mice bear the H-2 haplotypes which are generally associated with high responses to bovine gamma globulin and type 3 pneumococcal polysaccharide. A/J has been reported, however, to produce higher levels of antibody than C57 Br/cdj mice against the antigens. The two strains of mice were used as model systems in this study to determine whether the level of lymphoid cell surface proteolytic activity is also genetically controlled. The results of this study illustrate that the level of lymphoid cell surface proteolytic activity is strain related. Since A/J lymphocytes were found to have a significantly higher rate of proteolysis than C57 Br/cdj lymphocytes, a correlation between lymphoid cell surface caseinolytic activity and immune responsiveness is suggested.