At low concentrations, dimethyl sulfoxide (DMSO) stimulated the avian myeloblastosis virus reverse transcriptase activity. About 40% stimulation was obtained in the presence of 5% (v/v) DMSO, using activated DNA and polyriboadenylic acid (poly(rA)) as templates, and Mg2+ as divalent cation. A similar stimulation by DMSO was observed with Mn2+ for the poly(rA)-dependent reverse transcriptase activity. DMSO at concentrations higher than 15% inhibited the reverse transcriptase reactions, independent of the template-primers used. An exception was detected with the 2'-fluoro analog of poly(rA) as template, where an activation of 100% was found in the presence of 20% DMSO. The stimulation caused by DMSO could be due to a reduction of the apparent Km value for poly(rA) from 9.1 to 3.3 micrograms/ml.28285-90