Procollagens differ from collagens in that they have propeptide extensions at the amino and the carboxyl termini of the collagen α-chains. This chapter describes the isolation of Type I and Type II procollagens and their propeptides. It discusses the development and use of specific antisera against these propeptides. Chick Type I procollagen can be conveniently isolated from the media of chick tendon fibroblasts in suspension culture. Chick Type II procollagen is isolated from chondroblasts in suspension culture. Chick Type I propeptides can be isolated in relatively large amounts from leg tendons in tissue culture. Type II propeptides are isolated from the media of chick sterna in tissue culture by a modification of the organ culture system. The isolation of propeptides of Type I and Type II procollagens as described here allows the production of antibodies that are specific for the N- and C-propeptides.