If one accepts the minimum requirement of 4 i.u. shd-1 for the potency of pertussis vaccine--and there is no convincing evidence to reject this--the usual MPT with 20 mice per dilution offers a sufficient guarantee that no vaccines with too low potency will be accepted, provided that the consistency in production has been sufficiently proven. As higher concentrations than necessary should be avoided in view of the undesirable side-effects, the establishment of requirements for the lower level of the 95% confidence limits might, however, entail exactly this risk.