Infections caused by commensal microorganisms, such as Candida albicans often represent a severe complication in pharmacologically immunosuppressed kidney transplanted patients. A mannoprotein (MP) antigenic preparation derived from the C. albicans cell wall was used to measure the specific immune responsiveness in 44 kidney transplanted patients and matched healthy controls. Patients immune responses were analyzed considering the transplant age. In group I, patients transplanted from greater than 1 month to less than 12 months were considered, whereas in groups II and III patients had been transplanted 1 to 3 years or 4 to 6 years earlier. A statistically significant low responsiveness to MP was recorded in peripheral blood mononuclear cells (PBMC) from patients belonging to groups II and III. Addition of exogenous IL-2 to PBMC cultures restored MP-induced proliferation in about 50% of patients studied. Responsiveness to mitogenic stimulation (PHA and SEB) was in the normal range in all transplanted patients. No correlation could be detected between hyporesponsiveness to MP and C. albicans infections.