Malignant and benign human breast tumours as well as rabbit breast tissue were examined for specific receptors for 1,25-dihydroxyvitamin D3 using competitive binding studies and sucrose density gradient analysis. Classical high affinity, low capacity receptors for 1,25-dihydroxyvitamin D3 were found in breast and node tissue in seven of ten patients with breast cancer and in all three patients with benign neoplasms. An inflammatory breast mass showed no binding. Similar receptors were found in breast tissue from pregnant and lactating rabbits. Taken with other recent data, these results suggest that 1,25-dihydroxyvitamin D3 may activate calcium transport in the malignant as well as in the normal lactating breast.