A sensitive radioreceptor assay (RRA) capable of detecting 0.2 ng of human growth hormone (hGH) has been developed using plasma membrane fragments derived from the liver of pregnant rabbits and 125I-hGH. Human, monkey, bovine and ovine GH all inhibit the binding of 125I-hGH, while the inhibition by hPL or human prolactin (hPRL) was minimal.