1. A fragment of human subcomponent C1q was prepared by limited proteolysis with pepsin at 37 degrees C for 20 h, and at pH4.4, followed by gel filtration on Sephadex G-200. This fragment was shown to contain all the collagen-like features known to be present in the intact molecule [Reid (1976) Biochem. J. 155, 5-17]. 2. Circular-dichroism studies showed the presence of positive bands at 230 and 223 nm in the intact subcomponent C1q and pepsin fragment respectively, compared with a positive band at 220 nm obtained for lathyritic rat skin collagen. These bands were abolished by collagenase treatment, which suggested that there may some collagen-like triple-helical structure in subcomponent C1q and that this structure resides in the pepsin-resistant portion of the molecule. However, the 230 and 223 nm bands had a substantially lower magnitude than that obtained for the unaggregated single fibres of totally triple-helical collagen. 3. Thermal-transition temperatures obtained for subcomponent C1q, the pepsin fragment and the reduced and alkylated pepsin fragment were 48 degrees, 48 degrees and 39 degrees C respectively, compared with a value of 38 degrees C obtained for lathyritic rat skin collagen. 4. Only the unreduced pepsin fragment regained significant amounts (up to 60%) of collagen-like structure, after heat denaturation and cooling, as estimated by circular-dichroism measurements. 5. Electron-microscopy studies of subcomponent C1q and the collagen-like pepsin-resistant fragment of subcomponent C1q showed that the six peripheral globular regions of the molecule were fragmented by pepsin leaving the six collagen-like connecting strands and fibril-like central portion intact.