The immunohistochemical (IHC) expression of PSA in tissue fragments has been analysed in 25 patients with benign prostatic hypertrophy (BPH) and in 50 patients with prostatic carcinoma. The fragments were obtained by transurethral resection and transvesical adenomectomy. Morphology was studied by hematoxylin-eosin and Gömöri's trichrome staining. For immunohistochemical study anti-PSA monoclonal antibodies (DAKO, EPOS) were used. In BPH cases a positive reaction in the cytoplasm of secretory luminal cells was noticed; the reaction was negative in basal and urothelial cells, in foci of squamous metaplasia and in the cells of mucous gland metaplasia. All the classic types of prostatic carcinoma, regardless of the degree of differentiation showed positive reaction with various intensity and distribution. A positive focal reaction was noticed in two ductal carcinomas of endometrioid type. The reaction was negative in transitional, epidermoid and small cell carcinoma. Though less useful in differentiating benign from malignant lesions, this method may contribute to the differential diagnosis of particular forms of prostatic tumors.