When cho1esterol-[4-l4C + 7/3-t] was used prac- tically all the tritium remained in the cholic acid molecule. Mild oxidation of the isolated cholic acid to its 7-keto derivative resulted in complete loss of the tritium label. Consequently, 7a- hydroxylation involves displacement of the 7a- hydrogen with at least 9370 and possibly complete specificity. The same stereochemical course has been observed for the hydroxylation of steroids at C~I, ie, displacement with retention of ...