Collagen produced in vitro by bone cells isolated from 19 patients with different forms of osteogenesis imperfecta (OI) was analysed. Clinically, four patients were classified as OI type I, 10 patients as OI type III and five patients as OI type IV. Bone cells of 12 of the 19 OI patients produced structurally abnormal type I collagen. Electrophoretically uniformly slower migrating collagen type I alpha-chains were found in one case of OI type I, in seven cases of OI type III and in one case of OI type IV; two cultures of OI type III produced two different populations of collagen type I alpha-chains, and one culture of OI type IV showed reduction-sensitive dimer formation of alpha 1(I) chains, resulting from the inadequate incorporation of a cysteine residue into the triple helical domain of alpha 1(I). Quantitative analysis of collagen metabolism led to the distinction of two groups of cultured OI osteoblasts. In osteoblasts of OI type I, mainly production of collagen was decreased, whereas secretion, processing and pericellular accumulation of (pro)collagen type I was similar to that in control osteoblasts. In contrast, in osteoblasts of OI types III and IV, production as well as secretion, processing and pericellular accumulation of (pro)collagen type I were significantly decreased. Low levels of type I collagen were found irrespective of the presence or absence of structural abnormalities of collagen type I in all OI types.