For a number of years Hesler & Smith (196oa-b) and Smith & Hesler (1962) have been working on a monograph of the North American Species of Lactarius. During the course of this study, each of us, operating mostly in his own geographic area, has discovered many as yet undescribed species and observed and studied much variation among the supposedly well known species. I wish to take this special occasion to report on variation in two species which it has been my lot to study in detail. The study sheds considerable light on the taxonomic parameters of characters often used in delimiting species in this genus. Species common to both Europe and America have been selected since it is important to ascertain the number and type of variations occurring in each area before truly meaningful comparisons can be made. Since a point of departure is needed in order to discuss variation, a description of each species is given here approximately as it will appear in the monograph by Hesler & Smith. A discussion of the variants and variations follows. In the descriptions, the colour terms within quotation marks are from R. Ridgway, 'Color Standards and Color Nomenclature' (1912). Collections cited are in the University of Michigan Herbarium but the detailed citation of materials studied will be found in the forthcoming monograph by Hesler & Smith. One of the features that impresses me most about our Lactarius flora here in North America is the number of intermediates which occur 'between' species. A second feature is the variability in a number of characters such as taste, spore-deposit colour, spore size, colour change, and pigmentation of the pileus. Most important, however, is the variation in 'hyphal' characters, such as cell differentiation, from the common tubular type basic for the higher fungi. In other words, it is possible to overemphasize hyphal characters in taxonomy just as other characters have been overemphasized in the past. Relative to the problem of comparing the floras of North America and Europe, we need to know the 'spatter pattern' of variants around each species on both continents and to compare these patterns in our interpretations of the flora of each area. By ascertaining trends in the known variants of a species, indications of trends in the evolution of species will become apparent. The lesson seems to be that the mycologist cannot correctly claim to 'know' his species until he has ascertained the pattern of its variability.