SUMMARY: The binding of 5α-androstane-3α,17β-diol to human plasma proteins was studied by equilibrium dialysis. The mean value of 96·6% for men was significantly less than that of 97·5% for women. These values are higher than those found for 5α-dihydrotestosterone (95·0 and 97·3%, respectively) and for testosterone (92·4 and 95·3%, respectively). When the distribution of labelled steroid in the different protein fractions was studied by paper electrophoresis, male plasma showed a larger fraction of 5α-androstane-3α,17β-diol radioactivity in the albumin fraction and less in the β-globulin area than did female plasma. Similar finding were made when 5α-dihydrotestosterone was studied. No inter-α-globulin peak of radioactivity was found for 5α-androstane-3α,17β-diol and 5α-dihydrotestosterone as occurs for testosterone. Albumin binds more 5α-androstane-3α,17β-diol than 5α-dihydrotestosterone or testosterone when studied by equilibrium dialysis and this is due to a higher apparent association constant between albumin and the dihydroxysteroid.