An improved method for the simple determination of bromide ion in plasma is described. It was modified from the method by Kawai, Sugiyama et al. (1985). In the improved method, 200 microliters of plasma and 800 microliters of distilled water were placed in a reaction vial to which 100 microliters of dimethyl sulfate was added. The reaction vial was immediately sealed, placed in an incubator at 85 degrees C, and shaken for 10 min. At the end of the reaction, a gas-tight syringe was used to draw out the head space gas of 1 ml. The methyl bromide gas was then injected into a gas chromatograph and methyl bromide was determined with F.I.D. A good linear relationship was obtained between the peak area of methyl bromide and the concentration of bromide ion within the range of 0.5 to 500 ppm. Bromide ion in the range of 0.5 to 500 ppm in standard solution was added to the plasma samples. After 10 repeated measurements, the relative standard deviation was found to be in the range of 2.8-3.6% and recovery was over 96%. The analytical results of this method were compared with those obtained by spectrophotometry. The analytical results obtained by the two methods were in good agreement (r = 0.9669) in plasma samples of 10 methyl bromide workers. A study to determine bromide ion concentration in 39 methyl bromide workers was made and the results were compared with those observed in 100 workers (control group) not exposed to methyl bromide.(ABSTRACT TRUNCATED AT 250 WORDS)