Serum immunoreactive prolyl hydroxylase protein (S-IRPH), galactosylhydroxylysyl glucosyltransferase activity (S-GGT) and the amino-terminal propeptide of type III procollagen [S-Pro(III)-N-P] were studied in 24 patients with primary hepatocellular carcinoma, 18 with secondary liver neoplasms and 35 with other malignant diseases but no evidence of liver involvement; this latter group included 13 patients with Burkitt's lymphoma, II with breast cancer and 11 with other neoplasms. Control values were determined for 60 apparently healthy Nigerians. S-IRPH and S-GGT were above the upper normal limit, defined as the mean+ 2 SD of the controls, in all the patients with primary hepatocellular carcinoma and all but one with secondary liver neoplasms, whereas only one S-IRPH value and three S-GGT values exceeded this limit in the patients with other malignant diseases. The mean S-Pro(III)-N-P was even more elevated than S-IRPH and S-GGT in the primary and secondary liver neoplasm cases, but was also elevated in other malignant neoplasms; about one third of the patients with no evidence of liver involvement had a concentration exceeding the upper normal limit. A high correlation was found between the values for the three assays both in primary hepatocellular carcinoma and in the whole series of malignant diseases. The data suggest that primary and secondary malignant neoplasms of the liver have a high rate of collagen synthesis. The three assays may be of some value in the diagnosis of primary hepatocellular carcinoma and secondary liver involvement in other malignant diseases, and in monitoring the treatment provided.