A composition which comprises an animal cell population, and which contains immature animal cells. The immature animal cells are characterized by expression of alpha-fetoprotein or lack of essential expression of alpha-fetoprotein and albumin, and at least a portion of said immature animal cells or at least a portion of the progeny of said immature cells is capable of differentiating into cells which express albumin. The cell population is cultured under conditions which result in expansion of the cells. Expansion of the cells may be achieved by culturing the cells in the presence of an extracellular matrix and liver stromal cells; and preferably in the presence of growth factors. Such cells may be used for liver transplantation, artificial livers, and for toxicology and pharmacology studies. Such cells may also be genetically engineered to express proteins or polypeptides of interest.