Screening experiments with 30 selected purine and pyrimidine analogues have revealed five compounds which inhibited hemagglutinin production in mice. Of these, 6-mercaptopurine, 6-mercaptopurine riboside, 2-amino-6-benzylmercaptopurine and 2-amino-6-(1′-methyl-4′-nitroso-5′-imidazoyl) mercaptopurine were classed as slightly active. One of the compounds, thioguanine, was found to be highly active in toxic doses. 6-mercaptopurine did not inhibit hemagglutinin formation in rabbits given a dose of 6 mg/kg for 13 days when either human or sheep red blood cells were used as antigen. In groups of mice also, 6-MP, at a dose level of 25 mg/kg, failed to prevent hemagglutinin formation. Thioguanine, however, in doses of 5 and 2.5 mg/kg, but not 1 mg/kg, blocked primary hemagglutinin production. With single animals, the inhibiting effect of thioguanine was uniformly evident; with 6-MP, variations were observed among individual mice. The data are discussed in relation to previously published studies on 6-MP.