Increasing attention is being given to the use of adult rather than embryonic stem cells, both for research and for the development of transplantation treatments for human disease. In particular, mesenchymal bone marrow stem cells have been studied extensively because of their ability to self-renew and to give rise to various differentiated cell types, and because of the relative ease with which they can be obtained and cultured. In addition, the possibility of labelling stem cells with green fluorescent protein before transplantation has opened new and promising perspectives for their use in basic research. Because no structural or ultrastructural description of adult mesenchymal stem cells is available in the literature, this paper describes their morphology as revealed by light, confocal and electron microscopy, focusing on cells that are particularly suitable for transplantation studies, i.e. those derived from rat bone marrow transfected with green fluorescent protein. The results provide a basis for experimental studies of the differentiation of these cells in normal and pathological tissues.