Mononuclear cells from 39 patients with hypogammaglobulinemia of the common variable type were analyzed for in vitro proliferative response to a panel of cell activators in order to examine the lymphocyte response to mitogens and to study the capacity to generate an immunologically specific secondary response. Patient lymphocyte response to phytohemagglutinin and concanavalin A was found to be significantly lower than that of controls studied in parallel (P less than 0.01), and low response did not correlate with T-lymphocyte number. Response to pokeweed mitogen was significantly lower than that of controls (P less than 0.01), but response to zinc, tested in a few patients, was normal. Strong depressions of patient lymphocyte proliferative responses to Candida albicans, Escherichia coli, and Staphylococcus aureus were observed (P less than 0.01); all of these microbial activators require intact B-cell function for maximum response. Repeated testing of individual patients indicated that poor lymphocyte response could be consistently observed. Examination of change in vitro lymphocyte response during clinical course and disease management showed that a consistent pattern of intrinsic lymphocyte functional deficiency could be demonstrated.