The P blood-group system was discovered by Landsteiner and Levine in 1927. This study delineates: a) The ethnic group specific distribution patterns of the P1 blood group antigen in the population of Singapore. b) The occurrence rate of the anti-P1 antibody in the same population. In the blood donor population, the estimated incidence of the P1-negative phenotype was calculated to be 75%. Though the percentage of P1-negative individuals among the Chinese did not differ significantly from that for the Malays, it was significantly higher than that for the Indians (P less than 0.01). The weighted average incidence of anti-P1 in the blood-donor population over the period 1982-1987 was calculated to be 9.14 per 100,000. For the patients, the average incidence of the anti-P1 antibody was calculated to be about 13.9 per 100,000 patients. The Malays were noted to have the highest incidence of anti-P1 antibody despite the occurrence of a higher proportion of P1-negatives among the Chinese.