Serum beta-2-microglobulin (b2m) levels were studied in 10 patients treated with alpha interferon (IFN) for thrombocythaemia associated with chronic myeloproliferative diseases. In eight patients the IFN treatment decreased the blood platelet level to < 600 x 10(9)/l. Serum b2m levels rose significantly and dose-dependently immediately after the onset of treatment, by a median maximal increase of 72% with IFN 3 MU every other day, and 95% with 3 MU daily. The b2m rise caused by IFN abolishes the value of serum b2m in states where b2m is used as an indicator of disease activity.