It is unfortunate that there is conflicting immunological data relating to the practolol syndrome. It has not been established if "practolol patients" have some altered T cell function. The work of Behan et al. (1976) strongly supports this interpretation but other groups have not come to the same conclusion. The role of the antibody specific for a practolol metabolite needs to be determined. In practice this is going to be very difficult to do; the drug is no longer used and patients, for obvious reaons, are unwilling to take part in clinical experiments. It is likely that the nature of the metabolite will eventiually be defined and it would be a significant step forward if it could be shown that the antigenic determinant was not formed by other structurally related beta-blocking drugs. Meanwhile the stimulus to further research must be to produce an animal model system, but, for various political and economic reasons, support for such a research effort is difficult to justify.