Studies were conducted on the nature of nonspecific inhibitors of echovirus and reovirus hemagglutination (HA) present in human sera. It was found that the inhibitors are heat stable (in fact heating at 80°C increased their activity), and the inhibitory activity of human sera was not significantly reduced by treatment with individual proteolytic or lipolytic enzymes, RDE, periodate, protein denaturing agents or certain organic solvents. However, treatment with a combination of chymotrypsin and phospholipase C inactivated the inhibitors, suggesting that they might be a phospholipid-protein complex. It was also found that lecithins, cephalin and sphingomyelin inhibit HA by certain of the echoviruses and reovirus type 1; this inhibitory activity was abolished or reduced by treatment with phospholipase C or filtrates from a psychrophylic Pseudomonas species, but treatment with lecithinase D had little or no effect.