Summary Studies leading to the development and use of a one-dose potency test for poliomyelitis vaccine are described. The test is based on the measurement of neutralizing antibody in the serum of 6-week-old chickens that received one dose of a single dilution of vaccine. The basic principles which should be adhered to in the design of such a test are discussed. In studies using chickens 2 to 8 weeks of age, antibody response increased up to an age of 4 to 5 weeks and then leveled off. Trivalent vaccine diluted 1:10 or corresponding dilutions of monovalent vaccines were found to contain concentrations of antigen which would not saturate the antibody producing mechanism of the test animal and would thus permit effective measurement of vaccine potency. Antibody response was found to be specific in studies of cross-reactions of the three antigenic types, and in observations relating to possible modification of response to a given antigenic type as the result of trivalent combination. Practical application of the test is illustrated. Statistical criteria for the tests are discussed with comment on extension of the test for semiquantitative studies.