The rates of conversion of types I and III procollagens to their respective collagens were compared in seven fibroblast culture systems of murine or human origin. During 24 hr of radioactive labeling or after shorter pulses followed by chases of 20-48 hr, no evidence was obtained for conversion of radiolabeled type III procollagen to insoluble collagen. In the same cultures and under the same conditions, native collagen was generated from radiolabeled type I procollagen. There was no evidence for proteolytic degradation of type III procollagen during the chase experiments. The results are ascribed to a lack of availability of the enzymes required for processing of type III procollagen.