Levels of carcinoembryonic antigen (CEA) and glucose phosphate isomerase (GPI) have been compared in the circulating blood of hamsters bearing intra-muscular grafts of GW-39 human colonic tumour. CEA in the sera of GW-39 tumour-bearing hamsters ranged from 2-6 to 8-4 ng/ml (mean = 4-5 +/- 1-7 ng/ml). GPI in the sera of normal hamsters ranged from 332 to 749 iu/1 (mean = 602 +/- 110 iu/1) while those with 14-week-old intra-muscular grafts of a hamster amelanotic melanoma, (A.Mel.3), or GW-39 human colonic carcinoma had a range of 664 to 1267 iu/1 (mean = 1024 +/- 220 iu/1) and 1430 to 4719 iu/1 (mean = 2065 +/- 601 iu/1) respectively. Thus, the ratio of enzyme activity in GW-39, A.Mel.3, and normal hamsters was 3-4:1-7:1, indicating a significant elevation (P less than 0-01) in animals bearing a human colon carcinoma or a hamster melanoma, with particularly high values obtained in hamsters with GW-39. Sequential determinations of CEA and GPI in a group of hamsters transplanted intra-muscularly with GW-39 tumours revealed that both markers increased proportionately with duration of tumour growth, suggesting that both serum CEA and GPI may be used as measures of tumour growth. The concentration of GPI in GW-39 human colonic carcinoma xenografts was also significantly higher than that measured in normal human colon, primary human colonic cancer, or normal hamster tissues. These results support the view that GPI, in addition to CEA, is a quantitatively increased marker in this tumour model, and is liberated into the circulation in proportion to the increase in tumour mass.