The purpose of the 2 x 2 factorial study was to determine the effect of varying levels of dietary calcium (2.5 and 10 g/kg) and butterfat (50 and 200 g/kg) on lipid utilization and on development of colon tumours in animals initiated with 1,2-dimethylhydrazine dihydrochloride. Among rats fed on 200 g butterfat/kg, the fourfold increase in Ca intake induced more than a sevenfold increase in faecal excretion of total lipids and almost a fortyfold increase in faecal excretion of acid-extractable lipid. Among rats fed on 50 g butterfat/kg, the ingestion of supplemental Ca had a less dramatic effect and induced only a twofold increase in faecal excretion of total lipids and a threefold increase in acid-extractable lipid. The volume of intestinal adenocarcinomas was correlated with the excretion of acid-extractable lipid in faeces (R 0.369, P less than 0.02). Caecal enzymic activity was not correlated with tumour incidence or size or faecal lipid excretion. Overall, the fourfold increase in Ca intakes decreased total lipid absorption significantly but by less than 6%.