The usefulness of marsupial x eutherian somatic cell hybrids for studies of mammalian genetics and cell biology is discussed, and the problems encountered in obtaining such hybrids are outlined. In an initial investigation into these problems, the growth of marsupial cells in culture, and thegrowth and distribution of marsupial and eutherian cells in mixed culture were studied. Optimal conditions for mass and clonal growth of marsupial cells are reported. In mixed mass cultures, cell sorting occurred, but little inhibition of growth of either cell type was observed except in cultures which were nearly confluent. Colony formation by either cell type was greatly enhanced by growth on a feeder layer of marsupial or eutherian cells. Marsupial x eutherian hybrid cells are expected to grow successfully in the presence of excess parental cells.