An evaluation of an enzyme immunoassay for the determination of theophylline in serum (EMIT, Syva Corp.) is presented. The EMIT assay was fully mechanized by the use of an Eppendorf analyzer 5010. The precision of this enzyme immunoassay was satisfactory (coefficients of variation from day to day ranged from 5--6%). The EMIT assay proved to be specific, rapid and easy to perform. A comparison of the results obtained by EMIT and ultraviolet spectrophotometry in a series of 100 patients showed a good correlation between both methods.