Antiacetylsalicylyl antibodies were produced in rabbits and guinea-pigs by innoculation of acetylsalicylic acid incorporated in Freund adjuvant. These antibodies were readily detected by a passive hemagglutination test using rabbit erythrocytes incubated with acetylsalicylic acid at alkaline pH. Acetylsalicylyl conjugates with human gammaglobulin, bovine gammaglobulin and rabbit gammaglobulin were also prepared by incubating the proteins with acetylsalicylic acid at alkaline pH. In parallel experiments, salicylic acid did not induce formation of specific antibodies capable of reacting with acetylsalicylic or salicylic acid-treated erythrocytes.