Membrane-bound prolactin receptors from the mammary gland of 6-7-day postpartum lactating rabbits were solubilized using the zwitterionic detergent, Zwittergent 3-12 (3- dodecyldimethylammonio )-1-propanesulfonate). The solubilized receptor from one rabbit was bound to an ovine prolactin-agarose affinity gel and eluted at pH 4.2. The receptor appeared as a single band upon staining sodium dodecyl sulfate-polyacrylamide gels with Coomassie blue. The protein yield from one rabbit was about 8 micrograms and the overall yield of receptor was over 50%. The apparent molecular weight was 42,000 on sodium dodecyl sulfate gels but varied on molecular weight columns due to the type of detergent. Receptor inactivated by iodination had an apparent molecular weight of 21,000 on sodium dodecyl sulfate gels. The purified receptor did not bind to concanavalin A-agarose or Lens culinaris-agarose.