Pidotimod ((R)-3-[(S)-(5-oxo-2-pyrrolidinyl) carbonyl]-thiazolidine-4-carboxylic acid, PGT/1A, CAS 121808-62-6), a new biological response modifier, was investigated in 3 different pharmacokinetic experiments in healthy volunteers. A first trial was carried out with a cross-over design in 12 subjects, given the drug in single administration by intravenous route (200 mg in bolus) and by oral route at 3 dose levels: 200, 400 and 800 mg (tablets). The second experiment was performed in 36 subjects, by intramuscular route at 50, 100 and 200 mg (12 volunteers/group) twice a day for 15 days. Blood samples were drawn and urine collected at different times after the first and the last administration (29th) of the compound. The third experiment was done in 12 subjects given the product at the same single oral dose (800 mg) in different galenic formulations: sachets, vials and tablets, to assess the relative bioavailability, with a cross-over design. Pidotimod plasma and urinary levels were measured by HPLC. The plasma levels after parenteral administration followed a second-order pharmacokinetic, while after oral administration they were processed by a first order input-output model.(ABSTRACT TRUNCATED AT 250 WORDS)