A lethal chondrodysplasia characterized by extreme micromelia was diagnosed by ultrasound examination in two sibs whose nonconsanguineous parents were healthy. Radiographic and histopathologic data indicated that the two foetuses (18 and 21 weeks old) had achondrogenesis type IB (Fraccaro). Quantitation of total collagen extractable from dried cartilage samples demonstrated a 50% decrease when compared to an age-related control. This decrease was essentially related to type II collagen. Nevertheless, the alpha chains and the CB peptides of type II collagen had a normal electrophoretic mobility. A significant amount of collagen type I was also detected. The electrophoretic pattern of collagens type IX and XI did not differ significantly from control sample. The extracellular matrix elaborated by patient chondrocytes cultured in agarose for 10-12 days, contained less collagen type II than normal cells. Labelling with 14C-proline of cultured cells showed the presence of procollagen and type II collagen chains with a normal electrophoretic mobility, but an alpha 2(I) chain was detectable in the patient material, indicating the presence of collagen type I which supported the tissue findings. The significance of the type II collagen reduction in the patient's cartilage is unclear but it is unlikely to be the primary defect in achondrogenesis type I.