An enzyme immunoassay technique (EMIT) for microdeterminations of caffeine was compared with high performance liquid chromatography (HPLC) and evaluated in 113 neonates and young infants, and in 18 asthmatic and 15 epileptic children. The EMIT assay was found reliable in therapeutic drug monitoring. It offers advantages over HPLC in its rapidity and simplicity. It is not affected by hemolysis, hyperbilirubinemia, or lipemia. In the neonate, greater accuracy is obtained with blood samples containing no heparin.