IT was observed rather early in the history of the mouse histocompatibility-2 (H-2) system that some antigens of the system are limited to only one or very few H-2 alleles, whereas others are shared by several alleles1. Antigens with limited distribution were called “private”, as opposed to “general” antigens present in many different H-2 alleles2. It has been tacitly assumed that the distinction between private and general antigens (the latter will be called “public” here) is an artificial one and that the difference between, them would disappear when new strains and new H-2 alleles were analysed. This, however, proved not to be the case. So far H-2 alleles of some seventy strains and stocks have been determined, and the distinction into private and public antigens still holds. It can be argued, of course, that because most of the strains of the laboratory mouse are related to each other in their origin3, the differences in frequencies of individual H-2 antigens are still only apparent.