The anticarcinogenic effect of vitamin D3 in relation to biochemical and morphological markers in 7,12-dimethylbenz(alpha)anthracene (DMBA)-induced mammary carcinogenesis was investigated in two different sets of experiments. For each set, female Sprague-Dawley rats were divided into four groups, to allow comparison among treated and non-treated groups. At 50 days of age, animals of group B and C were given DMBA injection (0.5 mg/100 g body weight) through the tail vein, and normal control (group A) animals received the oil emulsion vehicle alone. Vitamin D3 at the dose of 0.3 microgram/0.1 ml propylene glycol was given orally twice a week, in carcinogen as well as non-carcinogen treated animals (group C and c), until the termination of the experiments (22-24 weeks for biochemical markers, and 35 weeks for morphology). At approximately 22-24 weeks, when marked lobular hyperplasia in DMBA control groups were confirmed through histology, the biochemical markers were modulated towards normal value for vitamin D3 in the treatment group, in comparison to the disturbed values caused by carcinogen administration in group B animals. Again, vitamin D3 supplementation was effective in reducing the tumour incidence (70% in comparison to 90% in group B). The results thus clearly concluded the antineoplastic potential of vitamin D3, and the existing correlation between biological and biochemical markers.