Isolation of the human malignant prostatic epithelial cell in pure monolayer culture was accomplished by clonal growth of single cells derived from metastatic deposits of prostatic carcinoma. Identification of these cells was established by the fact that the acid phosphatase of the prostatic epithelial cell is not inhibited by immersion in 10 per cent neutral formalin up to 24 hours, whereas all other acid phosphatases are destroyed by the formalin.