We examined 50 serum samples, submitted to our laboratories for theophylline levels, by the Enzyme Multiplied Immuno Technique (EMIT®), by a radioimmunoassay procedure, and by a high pressure liquid chromatographic assay. All three procedures classified patients similarly into subtherapeutic, therapeutic, and toxic levels. Precision and recovery values showed no significant differences between the three procedures. Precision ranged from 3.24 to 13.66%, while recovery values were from 98.6 to 110.0%. Significant differences were observed in the cost per assay and rate of throughput. The EMIT procedure was most costly and required the greatest amount of time. The excellent correlation between the Syva EMIT assay and the other two procedures makes this a desirable method for those laboratories which lack the more expensive instruments required for the other two procedures.