Guinea pigs were exposed by head only inhalation for 5 h to concentrations of approximately 1 ppm of tolylene diisocyanate (TDI) and 14 days later pinnal application of TDI on 3 consecutive days was followed by a dermal application of TDI after a further 7 days. The degree of contact sensitivity, measured 24 h later, showed that prior inhalation inhibited the skin reaction to TDI but it did not change the skin reaction to chlorodinitrobenzene (DNCB) in guinea pigs sensitised and challenged similarly with DNCB. The inhibition lasted up to 9 weeks. However, it did not occur in guinea pigs after intranasal instillation (instead of inhalation) and only occurred after oral dosage with TDI when the dose was relatively high. Treatment with cyclophosphamide before TDI inhalation reversed the inhibition of contact sensitivity, suggesting an involvement of suppressor cells.