Increased plasma sialidase activity was demonstrated in 25 patients with acute myocardial infarction. The mean plasma sialic acid level after incubation with substrate neuramin lactose was 0.060 +/- 0.003 (SEM) mumol/sample compared with 0.017 +/- 0.003 mumol/sample in 24 patients without infarction (p less than 0.001). The erythrocyte endothelial cell adhesion index was highest when both cell types were sialidase treated (0.35 vs control 0.00 [p less than 0.05]). When sialidase (50 U/ml) was added to whole blood in plastic tubes, the mean clotting time was 7.17 +/- 1.0 (SD) minutes (n = 8) compared with a mean control of 15.45 +/- 3.9 minutes (p less than 0.01). Sialidase had no effect on prothrombin or partial thromboplastin times. The above studies suggest that increased plasma sialidase activity in patients with acute myocardial infarction may be associated with clumps of desialylated erythrocytes that may alter flow in the microcirculation.