1. Strain differences of mice were found in the taste synergism between monosodium L-glutamate (MSG) and disodium 5'-guanylate (GMP). 2. Magnitudes of chorda tympani responses to the mixture of MSG and GMP over the sum of responses to each component were greater in the order of C3H/HeSlc(C3H) greater than C57BL/6CrSlc(C57BL) greater than BALB/cCrSlc(BALB) mice. The greatest synergism was observed in response to the mixture of 0.03 M MSG and 0.1 mM GMP, to which responses were about 2.6, 1.8 and 1.4 times greater than the sum of each component in C3H, C57BL and BALB mice, respectively. 3. Magnitudes of inhibition of MSG and mixture responses by the lingual treatment of proteolytic enzyme, Pronase E, were greater in the same order of C3H greater than C57BL greater than BALB mice as that observed in magnitudes of the synergism. These results suggest that there exists quantitative differences in receptors responsible for taste synergism between MSG and GMP among three mouse strains.