Our objectives are to find out whether the ouabain-resistant Li-Na resistant Li-Na countertransport system is present in the membranes of other human cells, such as lymphocytes and fibroblast and mouse neuroblastoma. We will investigate its characteristics and its relation to other modes of sodium transport, such as ouabain-sensitive Na/Na exchange, Na-coupled aminoacid accumulation NA/Ca exchange. We will study the Vmax of Li-Na countertransport in lymphocytes after initiation of cell division and differentiation by mitogen and antibodies. An important aspect to investigate is the presence of ouabain-resistant Na transport in lymphocytes cell lines selected for ouabain-resistance. We will characterize the kinetic and mode of operation of the ouabain-sensitive Na transport in these mutants.