We have previously demonstrated that the use of porcine mesenchymal stem cells (MSCs) isolated from the bone marrow can increase the proportion of somatic cell nuclear transfer (SCNT) embryos that develop to the blastocyst stage compared with adult fibroblasts obtained from the same animal (1). The aim of the present study was to determine if MSCs are also present in primary cultures of adult fibroblasts which are commonly used for cloning live animals. To do this we chose two primary adult cell lines that we have previously used to clone pigs. Single cells were isolated using low-density plating and then expanded. Cells were then differentiated to adipocytes, chondrocytes and osteocytes using protocols used previously by us for porcine MSCs (1). After seven days of culture, 57/90 (63%) of colonies for Line 1 displayed a typical fibroblast morphology, while the remainder appeared more rounded or cobblestone-like in appearance. For Line 2, 47/70 (67 %) of colonies had a fibroblast morphology while the remainder were cobblestone like. For Line 1, 2/90 clones differentiated into adipocytes (unipotent clones) and had a fibroblast morphology, 3/70 differentiated into chondrocytes and had a cobblestone morphology while 3 differentiated into adipocytes and chondrocytes but not osteocytes (bipotent clones) and had a cobblestone like morphology. For Line 2, 2/9 cell clones differentiated into adipocytes of which 5/9 were cobblestone like in appearance and 2/70 differentiated into adipocytes and chondrocytes and had cobblestone morphology. This is the first time to our knowledge that two different sub-populations of cells in primary adult fibroblast lines used to clone pigs have been described. Furthermore we have identified unipotent and bi-potent cell types in both cell lines. The significance of these findings for SCNT and the production of cloned animals remain to be determined.