Contrast agent safety during cardiac catheterization (CC) may relate in part to anti- or procoagulant effect. We studied these effects in 177 patients who underwent standard CC (N=112) or percutaneous transluminal coronary angioplasty (PTCA) (N=65) and received either iohexol (N=111) or ioxaglate (N=66). Patients received either 0 or 2000u heparin for CC or 10,000u for PTCA. The groups were similar in regards to age, sex, amount of contrast used, and procedure time. Partial thromboplastin time (PTT) and activated clotting time (ACT) were both significantly increased (P less than .01) in patients who received 10,000u heparin irrespective of type of contrast agent although larger increases were seen in the iohexol group. With 2000u of heparin, ACT and PTT increased significantly (P less than .01) only in the iohexol group. PTT and ACT actually decreased to similar and significant (P less than .01) degrees after both iohexol and ioxaglate when heparin was not used. We conclude: 1) commonly used measures of anticoagulant (ACT and PTT) show greater prolongation after either 2000 or 10,000u of heparin with iohexol than ioxaglate, 2) ACT and PCT appear to shorten with both iohexol and ioxaglate if no heparin is used. This data would suggest that ioxaglate does not have an anticoagulant advantage over iohexol.