A set of experimental model systems were designed to investigate (a) the inter-relationship between growth of two human cancer cell lines (SK-CO-1, HT-29) and carcino-embryonic antigen (CEA) kinetics; and (b) whether neoplastic growth or CEA concentration is modulated by human growth hormone (hGH). We found that increasing CEA concentration depended on tumour burden. SK-CO-1 cells had the lowest growth rates but the highest rates of CEA production. The rate of CEA increase exceeded the growth rate of both SK-CO-1 and HT-29. hGH modulated neither neoplastic growth nor CEA production. In conclusion, our results suggest that experimental models may be useful for investigating the role of serological markers as monitors of increasing tumour burden. It will be of interest to investigate the performance of those model systems in examining the effect of cytotoxic agents in neoplastic growth.