The vitamin D3 metabolites 1 alpha,24R,25- and 1 alpha,25S,26-trihydroxy vitamin D3 and their combinations with 1 alpha,25-dihydroxy vitamin D3 were tested for antirachitic activity in rats, chicken and Japanese quails. The trihydroxylated compounds were found to increase the activity of 1 alpha,25-dihydroxy vitamin D3. Since this synergistic effect is restricted to calcium absorption and bone formation while bone calcium mobilization is unchanged, the combined administration might improve calcium balance. These findings raise the possibility of a more efficient therapy of vitamin D-dependent diseases with limited amounts of 1 alpha,25-dihydroxy vitamin D3.