Interferon production was studied in mixtures of whole blood of healthy adults with tissue culture medium. There was no need to supplement the system with additional foreign serum and even the autologous serum and even the autologous serum could be removed by washing in serum-free medium with-out impairment of interferon production. Heparinized blood samples could be stored in the refrigerator overnight before performing the assay. In this test, production of interferon gamma was observed in response to phytohemagglutinin, concanavalin A, pokeweed mitogen, staphylococcal enterotoxin A, and to OKT-3, a monoclonal anti-T cell antibody. Production of interferon alpha was observed when viruses were used as inducers. Our experiments show that in the whole-blood assay the responses to several inducers of different types of interferons may be readily monitored under serum-free conditions. We believe that this test will be of value for testing large numbers of normal individuals (for investigations of the genetics of interferon production) or of patients with a variety of diseases.