Late allergic inflammatory reactions are probably of major importance for the development of asthma. In order to study the occurrence of early and late asthmatic reactions after challenge with different doses of allergen, inhalation provocation tests were performed in 13 patients with mild or moderate symptoms of allergic asthma. The provocation series was started with a low allergen dose (0.1-10 BU), which was then increased in successive ten-fold increments at intervals of 1 week until a pronounced bronchial reaction developed. Three different reaction patterns were observed. Six patients showed an isolated late reaction to relatively low doses of allergen. In four patients an immediate reaction was followed by a late reaction--a so-called dual response, and in three patients only an immediate reaction occurred. In four of the six patients who showed only a late reaction a higher allergen dose was given and this resulted in dual reactions in all four. One patient was challenged with an even higher dose, to which she reacted with an immediate response alone. After a late reaction, bronchial variability with low PEF values was observed over a period of several days. It is thus possible for an isolated late asthmatic reaction to be provoked by a low dose of inhaled allergen. This can be of clinical importance, repeated small doses of allergen may be unnoticed but still give bronchial inflammation and asthmatic symptoms.