The reproducibility of the early asthmatic response after bronchial challenge with cladosporium herbarum was studied in 5 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. Cladosporium herbarum was inhaled in the following concentrations (w/v): 1:100000, 1:10000, 1:1000 and up to 1:100. Histamine was inhaled in the following concentrations: 0.3, 0.6, 1.2 and up to 2.4 mg/ml FEV1 was measured before every inhalation and 2, 5, 10 min after. If FEV1 dropped greater than 20% after inhalation, the challenge was assessed as positive and the provocation on that day was stopped. The provocation was assessed as negative if this fall was not seen after the fourth inhalation. Two patients with positive skin test to cladosporium herbarum had reproducible early asthmatic response after inhalation of the antigen, they reacted to inhaled histamine and one of them presented an early asthmatic response after inhalation of saline. Three patients with negative skin test to cladosporium herbarum had an early asthmatic response after inhalation of the antigen on two out of six provocations, a response which was not reproducible. None of the three responded to saline. It is concluded that inhalation challenge with cladosporium herbarum may be more confusing than helpful in the diagnosis of early asthmatic response to this antigen, as patients with negative history and negative skin test sometimes present a positive bronchial challenge.