The taste of l-glutamate and its synergism with 5′-ribonucleotides is thought to be primarily mediated through the T1R1+T1R3 heterodimer in some mammals, including rodents and humans. While knockout (KO) mice lacking either receptor subunit show impaired sensitivity to a range of monosodium glutamate (MSG) concentrations mixed with 2.5 mM inosine 5′-monophosphate (IMP) in amiloride, wild-type (WT) controls can detect this IMP concentration, hindering direct comparison between genotypes. Moreover, some residual sensitivity persists in the KO group, suggesting that the remaining subunit could maintain a limited degree of function. Here, C57BL/6J, 129X1/SvJ, and T1R1+T1R3 double KO mice ( n = 16 each to start the experiment) were trained in a two-response operant task in gustometers and then tested for their ability to discriminate 100 µM amiloride from MSG (starting with 0.6 M) and IMP (starting with 2.5 mM) in amiloride (MSG+I+A). Testing continued with successive dilutions of both MSG and IMP (in amiloride). The two WT strains were similarly sensitive to MSG+I+A ( P > 0.8). KO mice, however, were significantly impaired relative to either WT strain ( P < 0.01), although they were able to detect the highest concentrations. Thus, normal detectability of MSG+I+A requires an intact T1R1+T1R3 receptor, without regard for allelic variation in the T1R3 gene between the WT strains. Nevertheless, residual sensitivity by the T1R1+T1R3 KO mice demonstrates that a T1R-independent mechanism can contribute to the detectability of high concentrations of this prototypical umami compound stimulus.