Carboxyfluorescein diacetate succinimidyl ester (CFSE) labelling of naïve lymphocyte populations provides unique insights into the immune response. The clonal nature of immune responses, necessitating clonal expansion to achieve a sufficiently large number of Ag-reactive effector cells, combined with the dependence of lymphocyte differentiation on cell division, underlie the usefulness of CFSE in understanding the factors that regulate responses both in vitro and in vivo. We have combined CFSE labelling with Ag receptor transgenic models, using seven channel flow cytometry to track the correlation between cell division and a number of other parameters, such as surface expression of activation markers, cytokine receptors and homing receptors, cytokine production, cytotoxic activity and indicators of apoptosis. Our data have allowed us to classify and understand immune responses in novel ways, suggesting many further avenues of enquiry and indicating previously unrecognized relationships between cell division and eventual cell fate.