Mice that received one intraperitoneal injection of 120 microgram of thymosin fraction 5 just prior to induction of interferon with polyriboinosinic-polyribocytidylic acid but did not have the production of interferon affected by the thymosin treatment. An additional treatment with 120 microgram of thymosin 24 h prior to exposure to the mice to polyinosinic-polycytidylic acid also had no effect on interferon induction. However, induction of gamma (type II immune) interferon in mice by tuberculin challenge of mice previously sensitized with Mycobacterium bovis strain bacillus Calmette-Guerin was affected by treatment of the mice with thymosin fraction 5. If the mice received 120 microgram of thymosin fraction 5 intraperitoneally 24 h prior to and again immediately before tuberculin challenge, a significant decrease in gamma interferon was observed. Thymosin treatment alone did not result in induction of interferon.