The anodic properties of the binding protein for vitamin D and its metabolites in human serum prompted development of a rocket immunoelectrophoresis assay for this alpha globulin. Only 0.2 ml of antiserum is required for assay of 72 samples in 5-.mu.l wells on 2 plates. The assay is sensitive (0.1 .mu.g) and precise (intra-assay CV [coefficient of variation] 1.4%; inter-assay CV 7.9%), and permanently recorded assay plates can be produced in 24 h. These features are attractive for the convenient measurement of this protein, which is important in anti-rachitic sterol transport.