The locus for the glycolytic enzyme phosphoglycerate kinase A (PGK-A) is sex-linked in the eastern grey kangaroo (M. giganteus) and the pretty-face or whip tail wallaby (M. parryi). Females older than the pouch life stage of development do not express their paternally derived allele in some tissues such as blood and liver. This phenomenon is called paternal X inactivation and is characteristic of kangaroos. This distinguishes them from eutherian females, which are mosaics of two kinds of cell brought about by random X inactivation of either the paternally derived or the maternally derived X chromosome. But in skeletal and cardiac muscle the paternally derived allele is weakly expressed. This paper reports data which show that in fibroblast cells cultured from tissue explants of these two species the paternally derived allele is also weakly expressed. This suggests the possibility of a form of mosaicism as in eutherians but with unequal proportions of the two types of cell. Our results show this is not so. Cloning experiments have been performed with cultured cells from three heterozygous M. giganteus females. A total of 49 clones was obtained and each clone was like its parent culture, i.e. there was a strongly active maternally derived allozyme and a weak paternally derived one. Two interpretations of these data are possible. Each cell may have a fully active maternally derived allele and a partly active paternally derived one. Or the clones themselves may be heterogeneous, such that one or the other of the X-linked alleles may switch on or off every few cell generations or less. Since X inactivation is known to be rather stable this latter hypothesis is considered the less likely. Autoradiography shows that the longer arm of the X chromosome in the cultured cells is late-replicating. Thus later replication of the X chromosome in kangaroos does not necessarily mean complete inactivity. Differences in morphology between the two X chromosomes in females are described.