Actived oxygen causes tissue damage in plants subjected to environmental stress or pathogen attack. Superoxide dismutase converts the highly reactive superoxide radical, formed when oxygen is activated, the peroxide. Peroxide is then detoxified by peroxidases and catalases. Cotton (Coker 312) was transformed by Agrobacterium tumefaciens carrying chimeric genes coding for a pea or a tobacco superoxide dismutase. Cell suspension were derived from callus formed on leaf tissue of primary transformants. Growth of the suspensions was determined under constant 30 C or a 12/12 h cycle of 30 C/15 C. Transgenic suspensions clearly maintained more normal growth rates and had a higher percentage of viable cells during chilling than the controls.