The reverse transcriptase (RT) of equine infectious anemia virus (EIAV) shares sequence similarity with the RTs of other lentiviruses, particularly with the RTs of human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2, respectively), the causative agents of acquired immunodeficiency syndrome (AIDS). There is a 41-42% sequence identity between EIAV RT and both HIV RTs (which have 61% sequence identity to each other). We have compared the enzymic properties of EIAV RT with those of HIV-1 RT. Several aspects of the activities of EIAV RT differ from the corresponding activities of HIV-1 RT. There are significant differences in the inhibition of the DNA polymerase activities by the deoxynucleoside triphosphate analogs, 3'-azido-2,3'-dideoxythymidine triphosphate, dideoxyTTP and dideoxyGTP and by the nonnucleoside inhibitor, tetrahydroimidazo[4,5,1-jk-1,4]benzodiazepin-2-(1H)-one and thione; in the dependence of DNA polymerase and RNase H activities on pH; in the inhibition of the DNA polymerase activities by the thiol-specific reagent N-ethylmaleimide; in the specific DNA polymerase activity; in the inhibition of the ribonuclease H activity by the zinc chelator orthophenanthroline. However, there are several cases in which EIAV RT and HIV-1 RT are more similar than was previously found for HIV-1 RT and HIV-2 RT. These include the Km values for the DNA polymerase activities, the heat stability of the DNA polymerase functions and the specific activity of the RNase H function.