The recently discovered murine ltk tyrosine kinase has been reported to resemble a transmembrane receptor devoid of an extracellular domain. We report here, however, that in in vitro experiments and in transfected cells, ltk uses an upstream non-AUG translational initiator and resembles a typical glycoprotein receptor, albeit a very small one, with a molecular mass of approximately 69 kd. Ltk is expressed at a very low level in only a few cell lines and tissues and may be the receptor for a pre-B lymphocyte growth or differentiation factor, because 10 tested pre-B lines each contained 2.4 and 2.8 kb mRNAs. By contrast, only one out of seven mature B cell lines expressed ltk and the in vitro maturation of pre-B into B cells was in one case accompanied by the inactivation of ltk expression. Ltk was also expressed in adult, but not in embryonic, mouse brain, and immunostaining detected the protein in essentially all neurons of the cerebral cortex and hippocampus, but not in the cerebellum.