A comparison of the rates of synthesis of collagen in normal skin, normal and hypertrophic scars, and keloids has been made by measuring the rate of incorporation of [14-C]-proline into peptide-bound [14-C]-hydroxyproline by tissue minces in vitro. The rate of synthesis of collagen, as measured by this technique, was significantly higher in skin than in normal scars whether the incorporation of radioactivity into hydroxyproline were expressed in terms of wet weight of tissue, weight of tissue DNA or weight of tissue hydroxyproline. The abnormal scar types exhibited similar rates of collagen synthesis, which were significantly higher than the rate in normal scars. Although the rates in both abnormal scar types appeared to be similar to that in normal skin when expressed in terms of wet weight of tissue, and weight of tissue hydroxyproline, they were seen to be lower than in skin in terms of weight of tissue DNA. The rate of synthesis of proteins generally, as measured by total radioactivity in non-diffusible peptides, was highest in normal skin and hypertrophic scar and lowest in keloid. The ratio of radioactivity in non-diffusible hydroxyproline to total non-diffusible radioactivity was almost twice as high in keloid as in normal scar, with intermediate values being observed in hypertrophic scar and normal skin. This indicated that collagen accounted for a higher proportion of the proteins being synthesised in keloid than in normal scar. The results confirm previous conclusions, from determination of the activity of the enzyme collagen proline hydroxylase, that the excessive accumulation of collagen in hypertrophic scars and keloids may, at least in part, be due to abnormally high rates of collagen synthesis in comparison to normal scars.