Polymorphism at Hbb (haemoglobin beta-chain) is widespread in natural populations of the house mouse, Mus musculus, and appears to be maintained by natural selection. This report is an attempt to correlate genotypic fluctuations at Hbb with a most important physiological attribute of haemoglobin, its oxygen carrying capacity. Oxygen affinity has been studied and P50 values have been measured in 12 inbred strains as well as wild-caught mice from Skokholm island. The mean P50 of each inbred strain is a constant characteristic, although there is high within-strain variation and the oxygen affinity of the blood of an individual can fluctuate considerably from week to week. The causes of this variation remain obscure but neither within-strain nor between-strain differences are correlated with known modulators of oxygen binding. In general, the blood of mice of inbred strains as well as wild-caught mice that are homozygous for Hbbd tends to have a higher oxygen affinity than that from comparable animals homozygous for Hbbs, but it seems likely that the oxygen dissociation properties of haemoglobin are not the only ones important in determining differential survival of a particular Hbb type under varying environmental stress.