A hormone-dependent subline of the transplantable rat mammary tumor MTW9 contains binding sites for both prolactin and estrogen. Prolactin binding is saturable (K-d similar to 2 times 10-9 M), hormone specific, and destroyed by proteases. By contrast, an autonomous subline derived from the same parent tumor has lost more than 75% of both prolactin- and estrogen-binding sites, although binding affinities for both hormones are unchanged. This reduction in binding sites for both prolactin and estrogen in the autonomous line may result in an incomplete recognition of the tumor cells as a target for the circulating hormones with a subsequent loss of hormone-dependent growth characteristics.