Antirickets activity of vitamin D3 and of its derivatives 1 alpha- and 25-hydroxy vitamin D3 1.25- and 24,25-dihydroxy vitamin D3 and 1,24,25-trihydroxy vitamin D3 was studied in chicken maintained on a diet containing various amount (0.01-2.40 mg) of strontium. Vitamin D3 as well as its 24-hydroxy and 24,25-dihydroxy derivatives lost their antirickets activity in presence of high strontium concentrations. Under the same conditions I alpha-hydroxy, 1,25-dihydroxy and 1,24,25-trihydroxy vitamins D3 retained the ability both to induce calcium-binding protein synthesis in intestinal epithelium and to stimulate bone tissue mineralization, although to a lesser degree as compared with controls, kept on strontium-free diet. These results suggest that presence of I alpha-OH-group in the steroid molecule is necessary for expression of maximal biological activity under conditions of elevated strontium content in diet while the activity of hydroxylase is inhibited in kidney. At the same time, the vitamin D3 derivatives containing the OH-group are responsible for impairments caused by increased accumulation of strontium.