Soluble class I molecules (sHLA-ABC) were measured by an enzyme-linked immunosorbent assay (ELISA) in plasma samples of 13 patients with chronic-phase Ph1-positive chronic myelogenous leukemia (CML). The patients were treated once daily with interferon (IFN) s.c. at a dosage of 4 x 10(6) IU/m2 IFN-alpha-2b or in combination with 50 micrograms IFN-gamma. Measurements were performed before 2, 4, 6, 8, 24, 48, and 72 h after the start of treatment and thereafter every 2-4 weeks. Baseline sHLA-ABC levels were within normal limits (mean 22.1 +/- 8.8 mg/l). An initial decrease of sHLA-ABC (mean 3.2 +/- 2.7 mg/l) was seen in all patients during the first 2-8 h of IFN treatment. Thereafter, sHLA-ABC levels increased steadily reaching maximum values within 2-5 weeks. The overall increase was 12.7 +/- 12.4 mg/l. During the following 2-4 months of IFN treatment sHLA-ABC decreased to near baseline levels in 12 of 13 patients. No difference was detected between IFN-alpha and IFN-alpha plus IFN-gamma treatment. beta 2-Microglobulin values were measured in 8 patients and were found to be correlated to sHLA-ABC concentrations (r = 0.48).