Abstract While preparing monoclonal antibodies against human lymphocyte surface antigens, we compared two strains of mice, which had been genetically selected by Biozzi et al. (1970) for their high and low antibody responses to sheep erythrocytes. Biozzi et al. (1975) subsequently showed that these high and low responder strains (HR and LR) have a high and low antibody response to almost any antigen. We observed with the HR strain a huge increase in the number of hybrids secreting an antibody directed against human lymphocytes.