We have examined the biological and biomechanical consequences of defective type II collagen production for fracture repair employing a genetically engineered mouse line Del1 which was generated by microinjection of a 39-kb mouse pro alpha 1(II) collagen gene construct containing a deletion of exon 7 and intron 7 (Metsäranta et al. [1992] J. Cell Biol. 118:203-212). Standardized tibial fractures were produced in transgenic Del1 mice and their nontransgenic littermates were used as controls. The fracture callus tissues were analyzed at days 7, 9, 14, 28, and 42 using radiography, histomorphometry, biomechanical testing, and Northern analysis of mRNAs for several tissue-specific matrix components. Deficient production of cartilage in Del1 mice resulted in reduced radiographic callus size, smaller cross-sectional area, and impaired biomechanical properties when compared with fractures of nontransgenic control mice. The differences were most evident in 14-day fracture calluses. Consequently mRNAs for cartilage-specific type IX and X collagens and aggrecan were also reduced in Del1 calluses. Levels of type II collagen mRNAs were unaffected since the mutated transgene produced additional type II collagen mRNA molecules. Further abnormalities in the fracture repair process of Del1 mice were observed in callus remodeling. In the control animals a typical feature of external callus remodeling was reduction of callus size during endochondral ossification between days 14 and 28. Such reduction was not observed in the transgenic mice. Histological examination of fracture calluses suggested also a reduction in trabecular surface area, which was found to be even more pronounced in metaphyseal bone of Del1 mice. Despite these differences the biomechanical properties of the calluses in the two groups became similar by day 28 of fracture healing. The results thus suggest that reduced chondrogenesis due to the presence of mutated transgenes in Del1 mice not only causes a temporary impairment in biomechanical properties of healing fractures but also affects later stages of callus remodeling.