Amongst the most important signal transduction molecules involved in regulating growth and differentiation are the protein tyrosine kinases (PTK). Since T cell development is a consequence of interactions between thymic stromal cells (TSC) and thymocytes, identification of the PTK in both compartments is required to dissect the mechanisms that control this process. Here we report a search for PTK in mouse TSC, using RT-PCR to survey the repertoire of PTK mRNAs expressed in a freshly isolated TSC preparation. We identified 10 different PTK cDNAs among the 216 cDNAs sequenced, and demonstrate that transcripts of three of those (ufo, fyn and fer) are widely expressed among a large panel of immortalized thymic epithelial cell lines (TEC) and in primary cultures of TSC. Of the other seven, none were expressed in established TEC lines but, instead, displayed distinct expression patterns in cell types likely to have contaminated the fresh TSC preparation, i.e., macrophages, B cells, T cells and fibroblasts. Among the three PTK expressed in TEC lines, only one, ufo, exhibited expression exclusively in cells of non-hemopoietic origin. Although expression of ufo (also known as tyro 7, axl or ark) is not thymic-specific, in that it is also expressed in cell types of mesodermal origin in other tissues, its presence in TEC suggests a role for ufo in differentiation of the TSC compartment. Consistent with this notion, high-level expression of this receptor PTK at the protein level could be documented in every TEC line investigated, as well as in fresh thymus tissue sections. These data provide the first example of a receptor PTK in TSC and open new approaches to study the regulation of TSC differentiation.