The activity of pidotimod ((R)-3-[(S)-(5-oxo2-pyrrolidinyl) carbonyl]-thiazolidine-4-carboxylic acid, PGT/1A, CAS 121808-62-6) on immunological parameters was evaluated in a double-blind trial, involving two Research Centres. 16 patients with a primary or metastatic neoplasm, 16 elderly patients under immunodeficiency conditions and 11 healthy volunteers were enrolled in the present study. The patients, randomized within each centre, were assigned to one of the following treatments lasting 15 days: one vial i.m. of pidotimod 50 mg, 100 mg, 200 mg twice a day, respectively; one vial i.m. of physiological saline twice a day. The lymphocyte PHA-stimulation test evidenced a significant variability due to the different treatment groups (p = 0.004). The analysis of the stimulation index (SI), computed from the mean c.p.m. before and after PHA-stimulation, showed a significant difference, dose-independent, between saline and active treatment (p = 0.002). The SI analysis, on the basis of the data of the allogenic stimulation test (mixed lymphocyte culture), confirmed the difference between saline and active treatment (p = 0.05) with a significant linear component in the time-effect curve (p = 0.001) but not in the dose-effect curve. A 12% increase in CD 3 lymphocytes compartment was observed with pidotimod 400 mg/day. The drug was well tolerated by all the patients included in the study.