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Insulin receptors in isolated cells from lactating mouse mammary gland were investigated by 125I-labeled insulin. A modified chloramin T-method after Hunter and Greenwood (1962) was used for insulin labelling with 125I; 125I-insulin with a high specific activity (80--120 muCi per microgram), high immunoreactivity and preserved biological properties was obtained. Three methods of labeled insulin purification were also studied: absorption on the cellulose column, gel chromatography on different types of Sephadex (G-25, G-75, G-100) and electrophoresis on polyacrylamide gel. The best conditons for iodination and purification of 125I-insulin suitable for studying the receptor-insulin binding in the isolated cells from mouse mammary gland were selected. Insulin binding to its receptor was found to be a specific reversible process, having high affinity and a tendency to saturation. The receptor-insulin-binding equilibrium was reached in 30 min at 24 degrees C. Specific binding was observed at a very low concentration of 125I-insulin (0.4.10(-9)M), close to the physiological level. Saturation was observed at a concentration of over 1.5.10(-9)M. Native, non-labeled insulin, at a concentration of 1 ng per ml lowered the labeled insulin binding by 10--20 per cent, whereas 10 ng per ml led to a 60 per cent lowering. The affinity constant of the process was about 10(9).M-1. Each cell had about 3 000 to 4 000 insulin binding sites.