1,25 (OH)2 D3 has been shown to be able to reduce the growth of several human cell lines. The effect of 1,25 (OH)2 D3 on the growth of breast cancer cell lines in relation to their oestrogen (ER) and progesterone (PGR) receptor content has been investigated. The growth inhibition of BT-20 and MCF-7 cell lines is related to the dose of 1,25 (OH)2 D3. It is dependent on the foetal calf serum concentration in the culture medium. At low concentration of 1,25 (OH)2 D3 the inhibitory effect is not detectable in the presence of 10% FCS. The rescue of cells from inhibition by serum was less effective when 1,25 (OH)2 D3 was present in the medium. The results of [3H]thymidine incorporation experiments and DNA measurements are in agreement with the reduction of cell number. Analysis in flow cytometry indicated a reduced number of cells in S phase. These data indicate that 1,25 (OH)2 D3 is able to modulate the growth of human breast adenocarcinoma cells regardless of their sex steroid dependency.