The interaction of rat liver sterol carrier protein (SCP) and 7-dehydrocholesterol or vitamin D3 was analyzed by the method of Scatchard plots and gel filtration on Sephadex G-100. Scatchard plots of binding data revealed that the binding behavior for 7-dehydrocholesterol was monophasic, while that for vitamin D3 was biphasic. In 7-dehydrocholesterol, the apparent number of binding sites and the apparent assocation constant K were 0.72 nmoles/mg protein and 8.75 X 10-7M-1, respectively. On the other hand, vitamin D3 showed two types of binding sites differing in affinity. The apparent number of binding sites and the K for high affinity binding were 0.65 nmoles/mg protein and 7.08 X 10-7M-1, those for low affinity binding were 1.51 nmoles/mg protein and 0.36 X 10-7M-1, respectively. Gel filtration of SCP on Sephadex G-100 column gave three protein peaks (peaks I, II and III protein according to the elution orders; Ve/Vo=1.0, 1.56 and 2.29, respectively). 7-Dehydrocholesterol was able to bind with peak III protein, while vitamin D3 was bound peaks II and III protein, respectively. The molecular weight of peak II protein was estimated to be 44,000 and that of peak III protein was 16,000. From these results, it was clearly demonstrated that SCP was involved in the transformation of 7-dehydrocholesterol to cholesterol and could also bind vitamin D3.