Abstract Alterations in nuclear magnetic resonance (NMR)-visible lipid, morphometric lipid volume fraction, distribution of subcellular lipid droplets and activation antigen expression were examined in human peripheral blood lymphocytes, activated using phorbol myristate acetate (PMA) and ionomycin or by co-culture with autologous monocytes. PMA/Ionomycin treatment caused significant time-dependent increases in mobile lipid and in oil red O-positive lipid droplets that were accompanied by lymphocyte proliferation and increases in activation antigens, CD25, CD69 and CD71. Co-culture of lymphocytes and monocytes also induced significant increases in NMR-visible lipid signals and cytoplasmic lipid droplets, but in contrast, no correspondent increases in activation antigens were observed. Strong correlations were observed between the intensity of the NMR signal and the percentage of total cells containing lipid droplets ( r=0.95) and the morphometric lymphocyte lipid volume fraction ( r=0.80), indicating that the droplets were the source of the mobile lipid signal. Lipid droplets in PMA/Ionomycin-treated cells were evenly distributed throughout the population, but in co-cultures, only lymphocytes in close proximity to monocytes with lipid droplets contained oil red O-positive lipid. This data shows that the NMR-visible mobile lipid signal observed in lymphocytes co-cultured with monocytes is not directly dependent on either proliferation or the upregulation of activation antigens, similar to the previously observed response of T cells exposed to antibodies to the T cell receptor.