Cholografin and Renografin 76 were studied to determine their effects on platelet function. In vitro platelet aggregation was significantly inhibited by at least 3.4 micron/ml Cholografin and 19.5 micron/ml Renografin 76. Patients who received Cholografin for intravenous cholangiography, and Renografin 76 for non-cardiac angiography, had low levels of plasma contrast agent, and hemostasis was clinically unimpaired. Patients who received Renografin 76 for cardiac angiography had inhibition of platelet aggregation at high levels of plasma contrast agent; there was no correlation with prolonged bleeding times, or with bleeding complications. High levels of plasma contrast agent may inhibit platelet aggregation in vitro and in vivo, although this may not be associated with clinically significant bleeding.