The acid phosphatase (AcP) isoenzyme in a human prostatic cancer cell line was compared to that of prostatic tissue extract by electrophoresis. The major isoenzyme by prostatic tissue extract is the AcP isoenzyme 2, while only AcP isoenzyme 4 (AcP-4) was observed in the human prostatic cancer cell line. A monoclonal antibody specific to AcP-4 was used to investigate the ultrastructural distribution of AcP-4 in a prostatic cancer cell line. The peroxidase staining pattern indicates that AcP-4 is synthesized on bound ribosomes, discharged into the cisternae of rough endoplasmic reticulum, transported to the cisternae of Golgi apparatus for concentration and packaging, and transferred to the secretory vesicles for exocytosis. It is well known that synthesis and secretion of AcP-2 are the major characteristics of the highly differentiated prostatic epithelial cells. The present data demonstrate the loss of this specific function in the prostatic cancer cell line. Instead of AcP-2, the dedifferentiated cancer cell line synthesizes and secretes AcP-4, which is a common AcP isoenzyme of many nonprostatic tissues.