The classification of nucleoside transport (NT) processes is based on functional and pharmacological characteristics including: transport mechanism, ' e' = equilibrative, 'c' = concentrative; sensitivity to nitrobenzylmercaptopurine riboside (NBMPR), a transport inhibitor, ' s' = sensitive, and 'i' = insensitive; and permeant selectivity. In this thesis we examined cytidine and guanosine transport in NB4 cells and characterized a novel 'c'oncentrative, Na +-dependent, 's'ensitive transporter that exhibits a preference for guanosine, thus named 'csg'. HL-60, acute myelocytic leukemia cells do not, while L1210, murine acute lymphocytic leukemia cells do exhibit 'csg' transport activity. It appears that concentrative sensitive guanosine transport is not unique to human leukemia cell lines and the presence of the 'csg' transporter suggests an important role for guanosine in particular forms of leukemia. NB4 cells exhibit bilineage potential with the ability to mature along the monocytic and granulocytic pathways of differentiation. I present data showing 'all'-trans retinoic acid (ATRA)-induced granulocytic differentiation of NB4 cells was accompanied by differential regulation of ' csg' and 'es' NT systems as demonstrated by nucleoside-specific changes in transport rates. Evidence for the presence of two classes of high affinity NBMPR binding sites was demonstrated in all NB4 membrane fractions studied. [3H]-NBMPR equilibrium binding assays are unable to distinguish between 'es' and 'csg' transporters. In this thesis we present evidence to support the possibility that one of the two predicted NBMPR binding sites is representative of 'csg' transporters. The lethal effects of nucleosides in several tissues have been known for many years. In this thesis we demonstrate that NB4 cells exposed to excess extracellular guanosine undergo cell death, via the process of apoptosis, in a concentration- and time-dependent manner. Of particular interest is the demonstrated cooperation of 1-beta-D-arabinofuranosylcytosine (ara-C) and guanosine in eliciting apoptosis in NB4 cells, the clinical relevance of which is discussed. (Abstract shortened by UMI.)