Comparative morphological and biochemical studies of primary cultures of parenchymal liver cells from adult rat liver cultured on collagen-coated plates and floating collagen membranes indicate that the latter have a markedly prolonged viability and show morphological and functional features reminiscent of liver in vivo. Tyrosine aminotransferase induction by cyclic nucleotides or steroids can be maintained in liver cells cultured on floating collagen membranes for periods of 3 weeks. The advantages of this system over those previously reported for the cultivation of liver cells is discussed.