The reproducibility of the late asthmatic response after bronchial challenge with cladosporium herbarum was studied in five adult asthmatics. Four bronchial challenges were done in randomized order with an interval of one week. The four challenges were: cladosporium herbarum, cladosporium herbarum, histamine and saline. The bronchial challenges were done double-blind. Up to four inhalations of the substance were given on one day. Cladosporium herbarum was inhaled in concentrations of 1:100000, 1:10000, 1:1000 and up to 1:100 w/v. Histamine was inhaled at concentrations of 0.3, 0.6, 1.2 and up to 2.4 mg/ml. The provocation was stopped if FEV1 dropped greater than 20% up to 10 min after the inhalation, or after the four inhalations. Ventilatory capacity was measured with one h intervals up to six h after the last inhalation. One patient with positive skin test to cladosporium herbarum had no late asthmatic reaction after inhaling a dose of antigen which gave an early asthmatic response. One patient with positive skin test and early asthmatic response after inhalation of cladosporium presented late asthmatic response after one provocation, but this response was not reproducible. None of the two patients with negative skin test had late asthmatic response after inhalation of the antigen, nor had they late asthmatic response after inhalation of saline or histamine. One patient with negative skin test and no early asthmatic response to inhaled cladosporium, showed reproducible late asthmatic response after antigen inhalation. This patient also presented late asthmatic response after saline inhalation, but not after histamine inhalation. It is concluded that late asthmatic response should be defined better than of fall from baseline greater than 20%.