Abstract 1,25 (OH) 2 D 3 has been shown to be able to reduce the growth of several human cell lines. The effect of 1,25 (OH) 2 d 3 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) 2D 3. It is dependent on the foetal calf serum concentration in the culture medium. At low concentration of 1,25 (OH) 2 D 3 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 D 3 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 d 3 is able to modulate the growth of human breast adenocarcinoma cells regardless of their sex steroid dependency.