Lipotropin (LPH) has been evaluated as a potential tumor marker using a sensitive beta melanocyte-stimulating hormone (beta MSH) radioimmunoassay. All 79 acetic acid extracts of carcinomas of lung, colon, stomach, esophagus and breast contained LPH in concentrations greater than blood; 61 of 79 extracts contained LPH in larger amounts than control tissues from patients without cancer. In a blind prospective study, plasma LPH was quantified in 107 patients admitted for work-up because of an abnormality on a chest roentgenogram. Thirty-one of 33 patients subsequently diagnosed as having benign lesions had plasma LPH within the 95 per cent confidence limits of normal subjects whereas 28 (36 per cent) of the 74 patients subsequently diagnosed histologically as having primary lung carcinoma had elevated levels. In control studies, 13 of 100 patients with chronic obstructive pulmonary disease had elevated plasma LPH levels; three of the 13 with elevated levels and four with normal levels have been diagnosed, during the two years of follow-up, as having lung carcinoma. In control studies of 23 patients with granulomatous lung disease, 22 had normal levels of LPH. In those with carcinoma of the colon elevated plasma LPH levels were observed in two of 21 untreated patients and in 11 of 61 patients receiving noncurative chemotherapy. Elevated plasma LPH levels were also observed in 10 of 59 patients with breast cancer, eight of 28 with pancreatic cancer, eight of 22 with gastric or esophageal cancer, six of 16 with renal cancer, four of eight with prostatic cancer, one of seven with cervical cancer and one of six with ovarian cancer. We conclude, an elevated LPH level is frequently observed in blood and tumor tissue from patients with various types of carcinoma.