Cell sources for generation of articular cartilage ex vivo are limited. To explore options other than stem cells, dermal fibroblasts were tested for their developmental potential when cultured on the cartilage matrix proteoglycan, aggrecan. A previous study suggested such an effort would be successful (M. M. French et al., Journal of Cell Biology 145:1103–1115, 1999). The adult dermal fibroblast cell line, RAB-9, was used in these assays. While initial attempts to differentiate the cells were unsuccessful, after pretreatment with insulin growth factor one (IGF-I), the cells were able to differentiate in culture on aggrecan. After 24 h in culture on aggrecan, the majority of the cells formed dense aggregates reminiscent of condensing mesenchymal cells in development. At 1 week, these aggregates stained positively with both Safranin O and antibodies against collagen type II. This staining was maintained through the conclusion of the experiment at week 4. RT-PCR for collagen II supports the hypothesis that dermal fibroblasts can be triggered to differentiate by culture on cartilage matrix proteoglycans. A three-fold increase in collagen type II mRNA expression is seen when cells are cultured on aggrecan in comparison to controls. These results provide an initial step towards a cell source that may prove equally successful for the generation of cartilage in the laboratory.