The in vitro immunomodulatory properties of the hypoxanthine derivative N alpha-5(1.6-dihidro-6-oxo-9-purinyl)pentyloxy/carbonyl -L-arginine (PCF-39) are described. Data obtained with PHA in vitro indicate that PCF-39 exhibits immunopharmacologic activity in elderly and post-cytotoxic patients, while showing no significant activity in normal donors. Antigen induced proliferation was unaffected by PCF-39, while PMN induced chemiluminescence was significantly enhanced. PCF-39 can be considered a new drug in the family of the hypoxanthine derivatives endowed with immunomdulatory properties.