Steroid receptors, prostaglandin output and enzymatic activities were determined in explants derived from human endometrium exposed to natural interferon-beta (IFN-beta). Receptors and cell metabolism were evaluated before culturing the tissue fragments and after a 3-day treatment with varying concentrations of IFN-beta. Total steroid receptor levels were unchanged when explants were set up, but there was a redistribution of both estrogen and progesterone receptors (ER and PR). A decrease in cytoplasmic receptors corresponded to an increase in receptor molecules within the nucleus. Treatment with low concentrations of IFN-beta caused a significant enhancement (p < 0.05) of ER and PR in neoplastic endometrium. In basal conditions the ratio between prostaglandin F2 alpha (Pgf2 alpha) and prostaglandin E2 (PgE2) was higher in normal than in neoplastic endometrium. The addition of low concentrations of IFN-beta to the culture medium determined a significant increase (p < 0.02) in PgF2 alpha and a parallel increase in the above ratio in neoplastic tissue, while no variation was found in normal endometrium. Analysis of the results concerning the variations in hormone-related enzymatic activities due to IFN-B revealed a significant increase (p < 0.05) in 17 beta-hydroxy-steroid-dehydrogenase (17 beta-HSD) activity. The data presented here indicate that treatment with IFN-beta modifies those biological characteristics of neoplastic cells which are involved in hormone-responsiveness.