This report describes a model of steroid transport i n human plasma. The binding affinities of 21 endogenous steroids for both testosterone-binding globulin (TeBG) and corti-costeroid-binding globulin (CBG) were determined under equilibrium conditions using a solid phase method at physiological pH and temperature. A computer program was used to solve the complex equilibrium interactions between these steroids and TeBG, CBG, and albumin. In this manner, we calculated the plasma distribution of each steroid into TeBG-bound, CBG-bound, albumin-bound, and unbound fractions in normal men, normal women during both the follicular and luteal phases of the ovarian cycle, and women during the third trimester of a normal pregnancy.