Summary. A radiolabelled monoclonal antibody (NDOG,) directed against placental alkaline phosphatase (PLAP) was used in the radio‐immunodetection of ovarian carcinoma. Tumour deposits were successfully visualized in 11 of 15 patients and the abnormalities demonstrated were classified as focal or diffuse. Of the 11 patients, eight showed focal abnormalities alone and three had a diffuse abnormality, of which two also showed a focal abnormality. False‐positive results may occur not only due to uptake of 123I by gut mucosa and an inadequately blocked thyroid gland but also from activity in an incompletely emptied bladder. A false‐negative result occurred due to high background activity in the liver masking a known, discrete tumour deposit.