Dietary fat influences many aspects of immune function. Escherichia coli endotoxin is a potent stimulator of interleukin 1 production from macrophages. The present study examines the effect of feeding with fat diets rich (corn oil) and poor (coconut oil) in linoleate at high and low concentrations on responses to endotoxin. Spleen phosphatidylcholine linoleate contents were higher in the corn oil than in the coconut oil group and arachidonate concentrations were highest in the group fed a high concentration of corn oil. Coconut oil completely abolished the responses to endotoxin. The inhibitory effects of coconut oil could largely be due to reduced prostaglandin and leukotriene synthesis.