The Center for Biologics Evaluation and Research within the US Food and Drug Administration has prepared a new US Standard Pertussis Vaccine. Whole cell pertussis vaccine concentrate was diluted in 5% (w/v) sucrose and lyophilized. The preparation was tested for toxicity, sterility, heterogeneity and residual moisture. Based on data from an international collaborative study involving 11 laboratories, the potency was estimated in relation to the US Master Standard Pertussis Vaccine, Lot 4 and the International Standard for Pertussis Vaccine, Lot 2. The potency of the preparation was defined to be 90 units per ampoule. When reconstituted and stored according to instructions, no significant change in potency was observed in the 14 days following reconstitution. This material was shown to be suitable for a pertussis vaccine standard and accordingly it was designated as US Standard Pertussis Vaccine, Lot 11 on March 22, 1994.