A B S T R A C T Peripheral blood mononuclear cells from 46 systemic lupus erythematosus (SLE) patients showed reduced ability to proliferate in vitro in response to the soluble antigen, tetanus toxoid, as compared with 96 normal controls. Special studies of27 untreated SLE patients also revealed significantly decreased blastogenic responses to tetanus toxoid. In both the total and untreated SLE populations, decreased mean tetanus antibody titers also were found as compared with the control population. However, the reduction in antibody titer and blastogenic response was not strictly parallel.