Thymocytes are selected for expression of alpha beta T-cell antigen receptors (TCR) which recognize antigen in conjunction with self-major histocompatibility complex (MHC) molecules. In the thymus the restriction element is imprinted on radioresistant stromal elements and on cells of haematopoietic origin. In mice negative for beta 2-microglobulin that are devoid of mature cytotoxic T lymphocytes, we find that intrathymic injection of different fibroblasts causes the maturation of CD4-CD8+TCRhigh thymocytes with distinct patterns of TCR V beta distribution. Here we show that in TCR-transgenic mice, intrathymic injection of L cells expressing the selecting H-2Kb molecule (L-Kb cells) reconstitutes the maturation of thymocytes bearing the transgenic TCR, and that in normal B10.BR (H-2k) mice, H-2Kb molecules expressed on L-Kb cells lead to the development of T lymphocytes with recognition restricted to H-2Kb. A class I MHC restriction element can thus be selected by interaction with fibroblasts, that is, cells of other than epithelial or haematopoietic origin.