The major objective of our research is to elucidate biochemical changes in cell cultures during the transition from quiescence to replicative growth. The order and nature of biochemical events during the G1 stage of the cell cycle, remain obscure. We have focussed our efforts on three approaches: (1) the role of cytoplasmic microtubules in the regulation of the activity of various peptide growth factors, (2) changes in phosphocholine synthesis and (3) the use of 5-(18F)fluorouridine as a probe for tumor proliferation in vivo.