A study has been made on the speed of growth and respiratory activity of fibroblast cultures from control derma, cheloid (hypertrophic) scar and stabilized scar taken from human skin. The speed of growth and the efficiency of plaque formation of fibroblasts from cheloid scar were greater in comparison with those of fibroblasts from stabilized scar and were stimulated by the addition to the culture medium of the exudate from post-traumatic ulcer. Measurement of the contents of cytochromes showed a decrease in the content of cytochromes b562 and c + c1 in the fibroblast culture from both cheloid and stabilized scar as compared to the fibroblast culture from control derma. Cytochrome aa3 content did not show significant difference among the three types of fibroblast cultures. The respiratory activities supported by pyruvate plus malate, succinate or ascorbate plus N,N,N',N'-tetramethyl-p-phenylenediamine did not show, however, significant difference among the three fibroblast cultures. These observations show that the speed of growth of skin fibroblasts does not depend on the overall respiratory capacity. The exudate stimulated the activity of cytochrome c oxidase in fibroblasts from control derma, and cheloid scar. This effect and the accompanying stimulation of fibroblast growth might be correlated with the balance of oxygen free radicals.