In man the Sd^a antigen is present in most secretions with the greatest concentration in urine. There is little difference in the amount of Sd^a substance in the urine of newborn infants, pregnant women and normal adults but there is a much greater amount in the saliva of newborn infants than in adults. Approximately half the people whose red cells group as Sd(a-) secrete Sd^a substance in the urine. Anti-Sd^a antibody is present in 50% of Sd(a-) non-secretors of Sda substance, though most of these antibodies only react with the strongest Sd(a+) cells. The greatest Sda activity has been found in human meconium, guinea pig urine and guinea pig kidney. Activity can be detected in the urine of many animal species. The kidneys of 5 bird species did not contain Sd^a.