The morphological effect induced by 1,25 dihydroxycholecalciferol - 1,25(OH)2VitD3 - on the malignant human breast cell line BT.20 was studied. This cell line is devoid of oestrogen (ER) and progesterone (PGR) receptors. This effect, which requires treatment for at least 3 days, was evidenced by an increase in the cell projection surface, assessed on scanning electron microscopy (SEM) and by quantimetric analysis, for optimal final concentrations of 1.25(OH)2VitD3 in the medium of the order of 10(-11)M. The cells became more spread out and rounded with many junctional systems; there was occlusion of the intercellular space, but hardly any cells overlapped. The cytoskeleton was considerably developed, with microtubules running parallel to the cell surfaces associated with microfilaments. This positive action on cell differentiation was very similar to that noted regarding the reduction in growth under 1,25(OH)2VitD3 treatment. The two actions were, however, observed at different times and with different concentrations of 1,25(OH)2VitD3 in the medium.