Treatment of K-562(S) cells with human interferons (HulFN) alpha, beta, or gamma results in a modulation of hemoglobin (Hb) production. When K-562(S) cells, "induced" with butyric acid or hemin, are given low dosages of all three types of IFN, the percent benzidine-positive cells doubles. Treatment with low doses of IFN causes the acceleration and increased production of those Hb types (as shown by Cellogel analysis) that are already synthesized either in untreated or in "induced" cultures. These results were confirmed by using pure HulFN-beta, and were abrogated, for HulFN-alpha, in presence of its specific antiserum. In contrast, cultures of K-562 cells treated with either inducer and given more than 10(4) IU/ml of alpha- or beta-IFN show a dose-dependent decrease of hemoglobinization in the absence of cell death. The inhibitory effect was reversible upon removal of IFN and culture reseeding in IFN- and inducer-free medium. The significance of both sets of data is strengthened by the pronounced heterogeneity of K-562S cells with respect to their sensitivity to IFN treatment as evaluated by the establishment of the antiviral state. Apparently, one cell out of two is sensitive to IFN, which suggests that the magnitude of IFN effects described here may be larger than it appears to be from the data taken at face value.