Addition of the methyl donor S-adenosyl-L-methionine to membranes prepared from mammary glands of lactating mice results in increased binding of 25I-labeled human growth hormone to the lactogenic receptors. This stimulation is dose dependent and specific for S-adenosyl-L-methionine and is partially inhibited by simultaneous addition of S-adenosyl-homocysteine to the reaction. Pretreatment of the membranes with S-adenosyl-L-methionine for 30 min at 37 degrees C is sufficient to cause enhanced binding. Scatchard analysis shows that treatment with S-adenosyl-L-methionine results in an increase in the number of lactogenic binding sites without changing the apparent affinity constant for 125I-labeled human growth hormone. The increase in the number of binding sites is believed to be due to alteration in the phospholipid composition of the membrane because methylation of phospholipids is observed under these conditions.