Oligonucleotides, differentially tagged with fluorochromes, were used to determine whether the distribution of mRNAs encoding the major milk proteins is heterogeneous within the mammary gland of the tammar wallaby (Macropus eugenii). This method also allowed direct visualization of two species of mRNA within the same cell. Sections of early and late lactating glands of tammar wallabies were hybridized with oligonucleotides labelled with fluorescein isothiocyanate or rhodamine isothiocyanate either alone or in combination. The results support the hypothesis that milk secretion is an all-or-none process with all epithelial cells in a given alveolus producing the same suite of milk proteins. In tammar wallabies, a gene encoding a protein specific to the latter phase of lactation appears to be expressed in those cells already secreting the other major milk proteins.