The cellular and humoral immunity of 15 patients with recurrent urinary tract infections was assessed during infection. We determined the reactivity of peripheral blood lymphocytes to microbial antigens (PPD, streptolysin O, tetanus toxoid, Salmonella typhi and Escherichia coli), to allogenic cells, and to mitogens (phytohemagglutinin, concanavalin A, pokeweed mitogen and jacalin). T and B cells were counted by the rosette technique. We also evaluated serum immunoglobulins and C3 and C4 levels by radial immunoprecipitation and specific antibodies for the etiological agent by direct agglutination, hemagglutination and indirect immunofluorescence. Urinary antibodies were assessed by the antibody-coated bacteria test (ACB). Some patients were retested for cellular immune function after cure. No significant alteration of immunoglobulin levels was detected, except for two patients who showed elevated IgM levels. C3 and C4 were elevated in one and in five cases, respectively. Specific serum antibodies for the etiological agent were present in all patients but their levels did not differ from those of normal controls. In only five patients were urinary antibodies detected by the ACB test. Patients' peripheral blood lymphocyte reactivity was depressed during infection for most of the microbial antigens and mitogens tested and returned to normal levels after cure. T and B cell numbers were within the normal range.