Food restriction is known to modulate aging and age-associated immune functions in rodents. In an attempt to understand the mechanism(s) through which food restriction delays age-associated loss of certain immune functions, lipid composition of spleen cells as well as binding of spleen cells to interleukin-2 (IL-2) and insulin were analyzed in four month-old and 19-month-old ad libitum fed (AL) and food-restricted (FR) Fischer-344 male rats. The results revealed that although AL-fed rats did not show a difference in age-related changes for IL-2 and insulin binding, the number of binding sites were significantly increased in the spleen cells of 19-month-old FR animals when compared with those of the 19-month-old AL group. When spleen cell phospholipid fractions were analyzed for fatty acid composition, the spleen cells from FR animals consistently revealed higher linoleic acid (18:2) levels and significantly lower arachidonic acid (20:4) and long chain fatty acid, docosatetraenoic acid (22:4) levels in the phosphatidylcholine and phosphatidylethanolamine fractions than the spleen cells of the AL rats. Further, spleen cell plasma membranes of FR rats also exhibited similar changes showing higher 18:2 and lower 20:4 and 22:4 levels than the AL animals. Finally, spleen cells obtained from 19-month-old FR animals produced higher levels of IL-2 and lesser prostaglandin E2 when compared to 19-month-old AL animals. The above observations suggest that one of the mechanisms through which food restriction may delay the loss of age-associated immune functions is through modulation of the fatty acyl composition of phospholipid fractions of spleen cell membranes. This modification may facilitate binding of IL-2 and insulin to their receptors and thus may improve T cell proliferation and prevent or delay age-related loss in immune functions.