Human tryptophanyl-tRNA synthetase (Trp-RS, EC 6.1.1.2) can be separated from its mouse counterpart by Cellogel electrophoresis. Analysis of the presence or absence of human Trp-RS and other human enzyme markers in eleven independently derived cell lines of human-mouse somatic cell hybrids revealed that the expression of Trp-RS is correlated with the expression of human nucleoside phosphorylase (NP, EC 2.4.2.1). The syntenic relationship between Trp-RS and NP permits the assignment of the structural gene for Trp-RS to human chromosome 14. Karyotype and isozyme analysis of these hybrid clones rules out other linkage assignments.