Human leukocyte interferon was given as adjuvant treatment for osteosarcoma in 26 patients. Blood from these patients was studied at regular intervals for antibodies to various micro-organisms. A continuous record of clinically manifest infections during the course of interferon therapy was made in 23 of the 26 patients. Distant metastases arose in 14 patients - the metastasis group. The report presents comparisons between these patients and the non-metastasis group. Seroconversion in tests with any of the studied micro-organisms was not common. Most of the seroconversions were not associated with clinical symptoms. Seroconversion during interferon therapy was confined to patients in the metastasis group. In some cases there was a chronologic link with early evidence of metastasis. The results of the serologic investigations are discussed and are correlated to the course of interferon therapy and of the neoplastic disease.