Dermal fibroblasts from a fetus with perinatal lethal OI synthesized normal and abnormal type I procollagen molecules. The abnormal molecules contained one or two pro alpha 1 (I) chains in which glycine at position 847 in the triple helical region was substituted by arginine as the result of a de novo G-to-A transition in the first base of the glycine codon. The substitution resulted in increased posttranslational modification amino-terminal of the mutation site of all chains in molecules that contained one or more abnormal chains. Secretion of the overmodified molecules was impaired, and intracellular retention of molecules which contained two abnormal chains was greater than that of molecules which contained one abnormal chain. The thermal stability of molecules that contained two abnormal chains was markedly lower than that of molecules containing one abnormal chain. After cleavage of molecules with vertebrate collagenase, the thermal stability of the overmodified A fragments was greater than that of the normal molecules. Our findings indicate that the cell distinguishes three classes of molecules and suggest that these molecules differ depending on the number of abnormal chains in the trimer.