The examples of "synergism" reported in the taste literature fit the common definition of the term but may merely reflect normal additivity in the taste system. That is, the perceived intensities of these "synergistic" mixtures exceed the sums of the perceived intensities of the unmixed taste components (addition of perceived intensities), but do not exceed the perceived intensities predicted by adding along the expandng psychophysical functions which describe the unmixed taste components (stimulus addition). True synergism requires that the perceived intensity of a mixture be greater than that predicted by both types of additivity. Subjects used magnitude estimation to judge the perceived intensity of unmixed monosodium glutamate and disodium 5'-guanylate and mixtures of these two taste substances. The mixtures showed synergism according to both crtieria. The robustness of this phenomenon is striking; for all subjects the taste mixtures showed true synergism. True synergism in taste is an extremely rare phenomenon.