Eleven foundry workers who developed asthmatic symptoms were studied with inhalation provocation tests with formaldehyde and diphenylmethane diisocyanate (MDI). These latter two chemicals are components of PepSet used for making molds for casting. Six subjects showed specific asthmatic reactions to MDI challenge (more than 20% fall in FEV1), 2 had a combined immediate and late reaction, and 4 had a predominantly late reaction. Four of the 6 subjects experienced recurrent nocturnal asthmatic reactions after a single challenge with MDI that lasted as long as 7 days, and this was associated with a significant increase in bronchial reactivity to methacholine. One subject had an immediate irritant reaction to both MDI and formaldehyde, which was spontaneously reversed in a few minutes. None of the remaining 10 subjects had a positive reaction to formaldehyde challenge. The reactors to MDI showed more evidence of air-flow obstruction in their lung function measurements and had a greater bronchial sensitivity to methacholine than the nonreactors but there was no difference in age, smoking habits, or atopic status. Specific IgE antibodies to MDI-human serum albumin conjugate were detected in 2 subjects, a reactor and a nonreactor. Specific IgG anti-MDI antibodies were detected in 4 subjects, 2 who reacted and 2 who did not. No difference in the pattern of response to MDI inhalation was observed in relation to the different immunologic findings. We conclude that MDI is a cause of asthma in foundry workers.