Following reports that vitamin D sulfate is the major source of vitamin D activity in human milk, we investigated the presence of this compound in milk whey using a modification of techniques for the determination of vitamin D metabolites in plasma. Synthetic cholecalciferol sulfate, ergocalciferol sulfate an [3H]cholecalciferol sulfate were prepared by reacting radioactive cholecalciferol or nonradioactive cholecalciferol or ergocalciferol with sulfamic acid in pyridine. The products were purified sequentially by Sephadex LH-20 and high pressure liquid chromatography. The purified products were chromatographically homogeneous, exhibited an ultraviolet absorption spectrum identical to that of standard cholecalciferol, demonstrated a sulfonate ester linkage and upon saponification yielded the parent vitamin. Milk whey was extracted with methanol:methylene chloride (1:2 v/v) using [3H]cholecalciferol sulfate to estimate recovery of the compound. The extract was purified by chromatography on silica cartridges an reverse phase high pressure liquid chromatography and was quantitated by ultraviolet absorption (UV). Although added cholecalciferol sulfate was readily detected in human milk whey samples, no endogenous vitamin D sulfate was found (detection limit 1 ng/ml). The results indicate that vitamin D sulfate is not a major source of vitamin D activity in human milk.