The site known as "Vector" contains the smallpox virus as well as samples of the Ebola virus.
Authorities said the gas explosion sparked a fire in a Russian laboratory that stores viruses from smallpox to Ebola.
The explosion occurred during the renovation of a sanitary and hygienic room on the fifth floor of the state research center for virology and biotechnology - known as "Vector" - in Koltsovo, in the Novosibirsk region, the center said on Monday. The site hosted research on biological weapons during the Soviet era and is now one of the main centers for disease research in Russia.
One worker received third-degree burns. The fire with an area of 30 square meters was later extinguished.
The Russian authorities insist that there were no biohazardous substances in the room where the explosion occurred.
The smallpox virus exists in two places on Earth: in Vector and in another highly secure laboratory at the US CDC in Atlanta.
In 2004, a researcher died in a laboratory complex after accidentally stabbing himself with a needle with the Ebola virus.