Disclosed is a reverse transcriptase composition having improved storage stability, comprising a reverse transcriptase, an effective stabilizing amount of at least one organic stabilizing reagent selected from trehalose and a nucleic acid containing a transcriptional initiation site recognizable by the enzyme, and an effective stabilizing amount of a metal salt capable of producing bivalent positive ions in an aqueous solution of the metal salt. Also disclosed is a method for improving storage stability of a reverse transcriptase, which comprises adding the above-mentioned organic stabilizing reagent and metal salt to a reverse transcriptase. The composition of the present invention can be stably stored for a prolonged period of time at a temperature up to at least 4.degree. C. Further, by virtue of a relatively high temperature usable for stable storage, the viscosity of the composition can be advantageously maintained at a low level, so that it becomes possible to accurately dispense the composition by a quantity corresponding to a desired enzyme activity, thereby achieving high reproducibility in experiments using the reverse transcriptase. Therefore, in the determination of a virus, in which a reverse transcriptase activity is used as an index, the composition of the present invention can be advantageously used as a standard substance for determining the amount of virus.