Human benign prostatic hyperplasia (BPH) tissues were obtained from patients undergoing transurethral resection of the prostate and viable cells from these were successfully maintained in primary cultures grown on collagen gel. The prostatic origin of the cells was confirmed by the measurement of prostate specific acid phosphatase and by scanning and transmission electron microscopy before and after immunostaining with human prostate specific antigen-antibody. The cell cultures were treated with various interferons (IFNs), both in the presence and absence of testosterone propionate (TP), for 72 hours and the activities of seven enzymes of carbohydrate metabolism were estimated in the cytosolic fraction of the cells. Treatment with TP induced a significant decrease in the activity of alpha-glycerolphosphate dehydrogenase (alpha-GPDH). Using this enzyme activity as a marker, the effects of various types of IFNs were investigated. IFN-alpha (wellferon) increased the activity of the enzyme both in the presence of one microgram./ml. of TP and in its absence whereas IFN-gamma inhibited the activity under similar conditions. The effect of treatment with IFN-beta in the presence of TP was biphasic in that there was an increase in the activity of the enzyme at the lowest concentration while at higher concentrations an inhibition of enzymic activity was observed. In the absence of TP IFN-beta inhibited the activity. The significance of these findings in terms of the clinical usefulness of IFNs is discussed and it is postulated that IFN-alpha (wellferon) might be effective in the treatment of metastatic carcinoma of the prostate in selected patients.