A series of 24-homologated 1,25-dihydroxyvitamin D3 compounds have been chemically synthesized and studied with regard to their activity in inducing differentiation of human promyelocyte HL-60 cells to monocytes and in calcium mobilizing activity in vitamin D deficient rats. Homologation of 1,25-dihydroxyvitamin D3 or its delta 22 analogue by one or two carbons increases by 10-fold and three-carbon homologation reduces by half the activity in causing differentiation of HL-60. On the other hand, homologation causes a substantial decrease in in vivo calcium mobilization activity. The addition of each carbon at the 24-position decreases binding to the HL-60 receptor or rat intestinal receptor by 5-10-fold so that binding affinity of the trihomo compound for the receptors is 130 times less that of 1,25-dihydroxyvitamin D3. Thus, binding affinity for the receptor cannot account for the preferential activity of the 24-homologated compounds in inducing cell differentiation.