The visna viral RNA-dependent DNA polymerase has been resolved into two forms by affinity chromatography. Glycerine gradient centrifugation of the two forms showed that one form sedimented at 6.9 S corresponding to an apparent molecular weight of 135 000 and the other at 6.3 S corresponding to 118 000. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis of the two forms indicated that the 6.9 S enzyme is composed of 2 molecules of 68 000 mol. wt. chain and the 6.3 S is a single chain enzyme. The latter form has been identified as a glycoprotein. The 6.9 S form can be completely inactivated in 20 min at 45 degrees C, prefers poly(rC) over poly(rA) as template and has high efficiency in utilizing visna 70 S RNA as template. The 6.3 S form is stable at 45 degrees C, active with 70 S viral RNA as template, prefers poly(rA) over poly(rC), and requires higher concentration of Mn2+ (0.4 mM) for maximum activity than the 6.9 S form does (0.1 mM) with synthetic homopolymers as templates. However, both 6.9 S and 6.3 S forms prefer Mg2+ over Mn2+ regardless of the nature of the templates.