Fibroblasts from patients with aspartylglycosaminuria (AGU) and from age-matched healthy controls were studied in culture. The rate of synthesis of collagenous proteins was lower in AGU fibroblasts than in control cells despite the fact that the growth rates and growth patterns were similar. Qualitative differences in culture media proteins between AGU and control cultures were not revealed by gradient gel electrophoresis or by CM-cellulose chromatography after pepsin treatment. DEAE-cellulose chromatography of AGU culture media components showed that they contained more [3H]glucosamine-labelled glycoproteins than the control cultures. Decreased collagen synthesis may explain the connective tissue symptoms (e.g. skeletal deformations and susceptibility to hernias) frequently present in AGU patients. Products from the incomplete intracellular degradation of glycoproteins can interfere with collagen synthesis in AGU. Aspartylglycosaminuria might thus provide a model for studying the regulation of collagen synthesis.