Carefully controlled allergen inhalation tests were carried out in twelve subjects to provoke early asthmatic responses with a mean maximum FEV1 fall of 30.7 +/- 5.2% (mean +/- s.d.). Four subjects had additional late asthmatic responses with a maximum mean FEV1 fall of 21.0 +/- 5.9%. The tests were repeated at intervals of 7 days in an identical way, following inhalation of Sch1000 (80 microgram) and placebo, each given 45 min before the onset of the early asthmatic response. This dose of Sch1000 produced a marked and uniform inhibition of methacholine-induced bronchoconstriction in the same subjects. The allergen-induced responses were reproducible in eleven out of the twelve subjects; the coefficient of variation for the decrease in FEV1 in the early responses being +/- 7% and in the late response +/- 43%. Sch1000 produced a slight and variable inhibition of early asthmatic responses (P less than 0.02) and no inhibition of late asthmatic responses. We examined the relationship between the degree of inhibition of the early asthmatic response by Sch1000 and : (a) the degree of inhibition produced by Sch1000 on histamine- and methacholine-induced bronchoconstriction; (b) the level of non-specific bronchial reactivity measured by inhaled histamine and methacholine; and (c) the degree of bronchodilatation produced by Sch1000. No relationship was found.