Chinese hamster and mouse cytoplasmic glutamate-oxaloacetate transaminase (GOT-1, EC 2.6.1.1) were separated by isoelectric focusing of cell extracts on thin-layer polyacrylamide plates. The expression of mouse GOT-1 was correlated with the retention of mouse chromosomes and the expression of 16 marker isozymes in 77 Chinese hamster x mouse somatic cell hybrids. Mouse GOT-1 expression was discordant to the expression of isozyme genes assigned to mouse chromosomes 1, 2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 17, 18, and X. Furthermore it showed no concordant expression with mouse chromosomes 3, 13, and 15. The expression of mouse GOT-1 was highly concordant to mouse chromosome 19 in 21 hybrid clones analyzed karyotypically. Our data confirm by means of somatic cell genetics the chromosomal assignment of the mouse GOT-1 gene based on Mendelian genetics by Eicher, Reynolds, and Southard (3). The appearance of a heteropolymeric band in hybrid cells expressing mouse GOT-1 suggests that this enzyme molecule is probably composed of two subunits.