The present report, together with extraction, separation and purification procedures described previously [1], constitutes a sensitive method for determination of vitamin D and its main metabolites in 2 ml of human serum. By omitting vitamin D itself, 25-OHD, 24,25-(OH)2D, 25,26-(OH)2D and 1,25-(OH)2D could be determined in less than 0.5 ml serum. The quantitation methods described have detection limits of 1.8 pmol for vitamin D, 20 fmol for 25-OHD, 24,25-(OH)2D and 25,26-(OH)2D, and 1 fmol for 1,25-(OH)2D.