The evidence from pre-licensure studies does not suggest that there are clinically important differences in reactogenicity between acellular vaccines. The merits of different acellular products will therefore have to be compared on efficacy criteria. Ideally, acellular vaccines with the minimum antigen content necessary to ensure optimum protection should be used in order to avoid administration of superfluous antigens to children and to simplify licensing and batch release procedures. On the basis of the evidence so far available it seems unlikely that monocomponent pertussis toxin (PT) vaccines provide optimal protection and that multicomponent vaccines are needed to achieve a level of disease control that approaches that of a good whole-cell vaccine. It is unclear whether all two component vaccines containing PT and filamentous haemagglutinin (FHA) have similar efficacy but on the available evidence the safest option for policy makers would seem to be to use a vaccine with at least three components, PT + FHA + pertactin. There is now good evidence that the five component vaccine which contains agglutinogens 2 and 3 in addition to PT/FHA and pertactin provides the best protection and is the only acellular vaccine whose efficacy matches that of a good whole cell vaccine. However, the public health advantage of the five component vaccine over other acellular vaccines may not become apparent until they have been in routine use for some decades and their ability to protect against transmission as well as clinical pertussis has emerged. The decision to replace an effective whole-cell vaccine by an acellular vaccine for primary immunisation needs careful consideration. Apart from the probable sacrifice of efficacy for reduced reactogenicity (at least for vaccines which do not contain agglutinogens 2 and 3) there is the question of value for money and the ease with which acellular DTP vaccines can be combined with conjugate polysaccharide vaccines such as Haemophilus influenzae type b. Whatever the decision of policy makers, the need for continued follow up of trial cohorts and active surveillance of the efficacy and safety of those acellular vaccines that are introduced into routine use must be accorded a high priority.