Confluent cultures of human skin fibroblasts were maintained for 10 days with sphingosine labeled [3H]GM2. Labeled medium was then replaced with normal medium and the cells maintained for 42 days with weekly medium changes. Cells were harvested at regular intervals and cells, medium, and trypsin digest supernatant analyzed for [3H]GM2 and its metabolic products. The ganglioside can be membrane associated and removed by trypsin, or membrane incorporated and trypsin insensitive. The membrane incorporated material is apparently transported to the lysosomes slowly by membrane flow, where 80% of the cellular GM2 can be metabolized by day 42. [3H]GM2 as well as its metabolic products in control cells is continuously released into the medium, during which it can also become associated with the cell surface membrane. There is no detectable metabolism of the [3H]GM2 in GM2 gangliosidosis cell lines over the extended post-labeling period, indicating that there is no residual enzyme activity in these cells. Undegraded GM2 is continuously released into the medium and remains associated with the cell surface membrane as well.