The aim of our study is to find whether the percentage of RAST positives to isocyanates within a company is a reflection of the exposure of the workers. Specific IgE was sought in 199 workers from ten companies where isocyanates were used. Twenty of them were followed for some time (june 87 to september 90). A non-exposed reference group was used for comparison. Measurements were made by the Pharmacia RAST-ELISA Method. In some cases, controls were made by RAST-RIA and CAPS ELISA/RIA. Results show that the proportion of positive RAST varies from one organisation to another and is probably a measure of the type and intensity of the exposure; the improvement of the working conditions to reduce the ambient concentration reduces, in time, the proportion of positive RAST; there were no positive RAST in the reference group; the conditions of taking and storage of blood samples before analysis influences the result. In conclusion, measurement of specific IgE may be an useful alternative to measurement of ambient concentration for following workers exposed to isocyanates.