1. Homogenates of Chinese hamster fibroblasts have been fractionated by analytical centrifugation techniques. Lysosomes have been characterised and identified by the distinctive behaviour of specific marker constituents. 2. The acid hydrolases of hamster fibroblasts were located in two distinct organelles, characterised by differences in their equilibrium densities and rates of sedimentation. The lighter population of lysosomes with a modal density of 1.10 g cm-3, contained glycosidases, nucleases and acid phosphatase. The denser population, with a modal density of 1.15 g cm-3, contained proteases and arylsulphatase B. 3. Acid hydrolases in both populations showed similar degrees of latency and structure-linked sedimentability. 4. The heterophagic activity of hamster fibroblasts was investigated by growing the cells in the presence of [14C]sucrose, [14C]inulin and Triton WR 1339. These compounds are taken up by the cells and appear to be located in the denser population of lysosomes. 5. The low density of the lighter population of lysosomes appears to result from a high content of cholesterol esters and triacylglycerols. It is suggested that the lysosomes may be the site of storage of a considerable proportion of these lipids in cultured hamster cells.