β-Endorphin is an endogenous opioid considered to be a modulator of immune injury. We studied the binding of [125I]β-endorphin on cultured rat mesangial cells at 4 and 37°C. The results were analyzed by computer program (Ligand). Incubation of rat mesangial cells with unlabeled β-endorphin displaced [125I]β-endorphin in a concentration-dependent manner. The binding of [125I]β-endorphin was not affected by either opiate agonists or antagonists. Saturation studies at 37 °C revealed that β-endorphin binding was time dependent. Binding studies revealed the presence of a single class of high-affinity binding sites with an apparent Kd of 15.3 nM. The number of receptor sites was calculated as 8.48 × 105 sites/cell. Mesangial cells exposed to β-endorphin (10-6M) for 48 h showed enhanced incorporation of [3H]thymidine when compared to untreated cells (control, 23,228 ± 2,778 cpm/well vs. β-endorphin, 44,887 ± 4,259 cpm/well; p < 0.01). Our results show that mesangial cells carry a specific receptor for β-endorphin which may be linked to proliferation of me