RNA tumor viruses contain a characteristic RNA-dependent DNA polymerase (reverse transcriptase) which has been thought to be related to the induction of leukemia by this virus. A disturbance in a zinc-dependent enzyme system was first postulated to account for the demonstrated differences in zinc metabolism of normal and leukemic leukocytes [Vallee et al. in (1949) Acta Unio. Int. Contra Cancrum 6, 869 and (1950) Acta Unio. Int. Contra Cancrum 6, 1102]. In order to investigate the relationship between zinc and the initiation of leukemia in chickens by avian myeloblastosis virus, we have examined the metalloenzyme nature of its reverse transcriptase. The present data show that this protein is a zinc metalloenzyme demonstrating the postulated relationship between zinc and a leukemic process. Paucity of purified enzyme generated the design of a novel system of analysis incorporating microwave-induced emission spectrometry combined with gel exclusion chromatography. It provides precision, reproducibility, and remarkable limits of detection on mul samples containing 10(-12) to 10(-14) g-atoms of metal, and is thus orders of magnitude more sensitive than other methods. The chromatographic fraction with highest enzymatic activity contains 1.8 x 10(-11) g-atoms of zinc per 1.6 mug of protein, corresponding to either 1.8 or 2.0 g-atoms of zinc per mole of enzyme for a molecular weight previously determined either as 1.6 or 1.8 x 10(5). Copper, iron and manganese are absent, i.e., at or below the limits of detection, 10(-13) to 10(-14) g-atoms. Agents known to chelate zinc inhibit the enzyme, while their nonchelating isomers do not. The data underline the participation of zinc in nucleic acid metabolism and bear importantly upon the lesions that accompany leukemia and zinc deficiency.