Conditions have been developed for reverse transcription by detergent-disrupted virions of Moloney murine leukemia virus which permit synthesis of molecules that appear to be complete transcripts of the 35S RNA subunits. At limiting Mg2+ concentration, DNA is synthesized in good yield, up to a maximum size of about 2.4 X 10(6) daltons. DNA larger than 2 X 10(6) daltons, taken from alkaline sucrose gradients, has no detectable self-complementarity and was protected from digestion by S1 nuclease to an extent of 90% by annealing to 70S RNA. All size classes of DNA made in these reactions are primed with RNA, because all are initiated with a pApdAjunction. To produce such long molecules, it is necessary to keep the concentration of Mg2+ in the reaction mixture below the total concentration of deoxyribonucleoside triphosphates. Under these conditions, degradation of the RNA template is minimized. The rate of DNA synthesis is also slowed by 30 to 50%, but products longer than 5,000 nucleotides, which are not found otherwise, are completed between 3 and 6h of reaction.