SummaryOut of a total of 60 sex‐linked or probably sex‐linked genes in man, 34 have no known effects in the heterozygous condition, and in another 4 the known effects are of a humoral kind and hence cannot give any information concerning the Lyon hypothesis (l.h.). Of the remainder, 17 genes have known or suspected effects in heterozygous women which are multicellular in nature; though these could, in principle, give rise to the patchwork mosaic manifestation demanded by the l.h., there is not a single really plausible case amongst them and none which can be so interpreted without variousad hocassumptions. The claim that the heterozygous phenotype of several of these conditions supports the l.h. can thus be shown to be without foundation. In the remaining five conditions, effects on the cellular level have been identified. In one of them (Xg), there is no evidence for the existence of two red cell populations in heterozygous women. Evidence for the existence of two distinct cell populations comes mainly from G‐6‐PD deficiency and, in a less complete form, from agammaglobulinaemia and Hurler's syndrome. In the first of these, the ‘ mosaic’ is so fine grained as to be difficult to reconcile with the l.h., which postulates inactivation early in embryonic life. Moreover, there is evidence that this phenomenon is not confined to sex‐linked genes, but occurs similarly in autosomal conditions. It is concluded that the behaviour of sex‐linked genes in man (like that in other mammals) gives no support to the