We have fused many different combinations of marsupial and eutherian cells, in order to test their capacity to form proliferating marsupial x eutherian somatic cell hybrids. During the first week after fusion we were able to identify marsupial x eutherian synkaryons, a number of which had undergone extensive chromosome fragmentation. For some combinations of marsupial and eutherian cells one parental cell type could not be completely eliminated from the fused cultures by the various selective systems employed, making it difficult to distinguish possible hybrid cells from parental cells. However, many hybridizations yielded discrete colonies which could be isolated and further studied. The first hybrids to be confirmed were produced from fusions between Pseudocheirus peregrinus (Marsupialia) lymphocytes and cultured mouse cells from the line PG19. These hybrid cells possessed clearly identifiable marsupial and mouse chromosomes and enzyme markers. Certain other combinations of marsupial and eutherian cells yielded 'eutherian-like' colonies. Although initially slow growing, these cells came to resemble the eutherian parental cell type in morphology, growth rate and cloning efficiency. Some of these colonies were shown to be composed of variant eutherian cells. However, others were clearly marsupial x eutherian hybrids as they possessed enzyme markers of each parental cell type even although, in some cases, these cells possessed no identifiable marsupial chromosomes. All the marsupial x eutherian hybrids isolated showed either partial or complete loss of marsupial chromosomes. We discuss the difficulties associated with the derivation of marsupial x eutherian hybrids, the important features of these hybrids, and their potential use for further studies.