Abstract: A simple device functioning as a radiorespirometer is a petri dish with a modified cover for quantitating an amount of CO.sub.2 evolved by living cultured cells. The device is a plastic culture dish having a culture surface circumscribed by a continuous sidewall. A removable flat plastic cover has a flat inside face and an annular collar around the face that fits against a sidewall. The collar is maintained in substantially gas-tight relationship against the sidewall by an elastic band or tightly mating surfaces. An opening through the cover is sealed by a gas-impervious material, such as a plastic film, that can be penetrated by a needle to introduce acid into the dish for terminating culture growth and lysing the cells. A collector tube is removably secured to the inside face of the cover and contains a CO.sub.2 trapping material, such as hyamine (methylbenzethonium hydroxide). The culture medium is provided with a cell substrate that contains radioactive carbon, and radioactive CO.sub.