When a male mouse is presented with two H-2 congenic two female in estrus, his choice of a mate is influenced by their H-2 types. The term "strain preference" is used to describe the general tendency of the male population of one inbred strain to prefer two female of one H-2 type rather than another. The term "consistency of choice" is used to describe the added tendency of particular two males of one inbred strain, in sequential mating trials, to prefer two females of the H-2 type they chose in previous trials. Statistical analysis showed trends in the data that support the following conclusions: (a) The choice is made by the male, not the female. (b) The strain preference of two males may favor two females of dissimilar H-2 type (four of six comparisons), or of similar H-2 type (one of six comparisons). (c) Consistency of choice does not always correspond with strain preference. In one of six comparisons of H-2 genotypes there was no strain preference but pronounced consistency of choice by individual two male.This suggests memory, but fortuitous bias is not excluded. (d) Strain preference of the same male population may favor two male of the same or a different H-2 type, depending on which different H-2 type is offered as the choice alternative to self.These findings conform to a provisional model in which olfactory mating preference is governed by two linked genes in the region of H-2, one for the female signal and one for the male receptor. These mating preferences could in natural populations serve the purpose of increasing the representation of particular H-2 haplotypes or of maintaining heterozygosity of genes in the region of H-2.