The specific binding of [125-I]iodohCG to intact luteal cells obtained from bovine corpus luteum by enzymatic treatment, or to purified plasma membranes obtained from bovine corpus luteum has been compared. It was a saturable process with respect to the [125-]iodohCG concentration. Specific binding could be detected at concentrations as low as 1.8 ng of HCG per ml and saturation achieved at 92 ng/ml. The [125-I]iodohCG specifically bound to the luteal cells or to the plasma membranes was displaced by increasing concentrations of native hCG. Subunits hCGalpha and beta had respectively 200- and 800-fold less activity than hCG. Four to 10 times more ovine LH than hCG was required to displace an identical amount of bound [125-I]iodohCG. The binding of hCG to its receptor site was a function of time and temperature. The affinity of hCG for its receptor sites in luteal cells or plasma membranes of luteal cells was similar (dissociation constants of 5.3 and 3.8 times 10- minus 10 M, respectively). The number of sites per luteal cell was 5 times 10-4 and the capacity of plasma membranes to bind hCG was 140 fmol per mg of protein at saturation. The data does not, however, allow a comparison between the number of binding sites in the two preparations. It is concluded that the enzymatic treatment necessary to obtain a suspension of viable luteal cells does not affect the kinetic characteristics of the binding of hCG to receptor sites since they are similar to those of plasma membranes not treated with proteolytic enzymes.