The binding of 125I-angiotensin II to human blood cells was investigated. Blood was drawn from healthy volunteers and platelets prepared with minimal contamination of red cells and white blood cells (125l-angiotensin II by platelets could be demonstrated in the presence of various enzyme inhibitors. However, when incubated, with 1 mM diisopropylfluorophosphate (DFP) or 1 mg/ml bacitracin, no breakdown of 125I-angiotensin II could be detected. The amount of specifically bound 125I-angiotensin II increased linearly with the number of cells per tube. Binding reached a plateau within 90-120 min at 37°C, and was stable thereafter. Specific binding was reversible. No binding could be detected at 4°C. Specific binding of 125I-angiotensin II was saturable. Scatchard analysis of binding by platelets of healthy volunteers revealed one class of binding sites with an apparent Kd of 127 + 16 pM and a maximal binding capacity of 7.9 ± 1.5 binding sites per cell. Competitive displacement of 125I-angiotensin II binding by angiotensin ll-analogues showed a rank order of effectiveness. Unrelated peptides, e.g. bradykinin, vasopressin and enkephalin, did not displace specifically bound angiotensin II. Human mononuclear leucocytes were prepared by a Ficoll-isopaque gradient. However, these cells could not be used for studies of specific binding, since enzymatic degradation of 125l-angiotensin II could not be prevented despite addition of various enzyme inhibitors. Time-dependent uptake of 125I-angiotensin II showed no stable plateau. Thus our study shows specific binding of 125l-angiotensin II to human platelets. These cells might serve as an in vivo model of angiotensin II receptor regulation in clinical studies.