The binding of 125I-labelled insulin to human adipocytes was studied at 37 degrees C. The precipitability of the 125I-labelled insulin preparation (0.03 nmol/l) in trichloroacetic acid and the concentration of biologically active insulin (7.5 nmol/l) remained constant in buffer incubated with human adipocytes (100 microliter cells/ml suspension) for 30--60 minutes at 37 degrees C, whereas more than half of the insulin was inactivated by rat fat cells under the same conditions. A constant level of binding of 125I-labelled insulin (0.03 nmol/l) to human adipocytes was obtained after 45 minutes. The apparent dissociation constant of receptor binding was about 0.2 nmol/l as compared to about 2 nmol/l for rat adipocytes. Conversion of [UP14C]glucose to lipids was stimulated half-maximally by about 0.05 nmol/l of insulin (similar to rat adipocytes). Thus, half-maximal stimulation of human adipocytes was obtained with a recptor occupancy of about 20--30 per cent.