The selective angiotensin (ANG II) antagonists losartan (DuP 753) and PD 123319 have been shown to bind selectively to AT1 and AT2 subtypes, respectively. To characterize ANG II receptor subtypes in mesangial cells, washed membranes were incubated with 0.1 to 0.5 nM 125I-ANG II and increasing concentrations of competitors. The inhibition of 125I-ANG II binding by losartan and PD 123319 was biphasic, and LIGAND curve-fitting analysis revealed two populations of specific binding sites. One subpopulation comprised 86% of the total and showed high affinity for ANG II and losartan, but low affinity for the AT2 antagonists PD 123319 and CGP42112A, and thus appear identical to the recently cloned AT1 subtype. The remaining 14% of the sites showed nearly 100-fold lower affinity for losartan and 10,000-fold higher affinity for PD 123319 relative to AT1 sites. However, another AT2-selective antagonist, CGP42112A, showed little affinity for these sites. Both classes of binding sites were inhibited by guanosine 5'-O-(3-thiophosphate) and pertussis toxin treatment. We propose that there are two distinct G protein-coupled ANG II receptor subtypes (AT1A and AT1B) present in renal mesangial cells.