A monoclonal antibody (H317) with specificity for the heat-stable placental-type alkaline phosphatase (PI-ALP) isoenzyme has been used to investigate the occurrence of PI-ALP in patients with primary breast carcinoma. All pre-operative plasma samples were negative for PI-ALP in a sensitive solid-phase enzyme immunoassay with a lower limit of detection of 0.1 U/l. In contrast, using a peroxidase-anti-peroxidase staining technique on fixed tissue sections, as well as enzyme immunoassay on fresh tissue extracts, PI-ALP could be demonstrated in the carcinomatous tissue of all seven patients investigated.