After stimulation with select activating agents such as lipopolysaccharide (LPS) or recombinant interferon-gamma (rIFNgamma), several macrophage proteins may be induced, acylated with myristic acid, or both. Our goal in this study was to determine whether altering the levels of myristic acid in the diet would modulate the levels of a specific acylated macrophage protein, MacMARCKS (myristoylated, alanine-rich C kinase substrate), because that fatty acid can be found in substantial quantities in some foods. Thioglycollate-elicited peritoneal macrophages from groups of mice fed diets with various levels of myristic acid (from 0.2 to 99 g/100 g fatty acids) were treated with LPS, phorbol myristate acetate (PMA), or rIFNgamma plus LPS, which are well-established macrophage activating agents. Levels of MacMARCKS were measured by enzyme-linked immunosorbent assay using a rabbit anti-mouse polyclonal antibody against the first 10 amino acids of murine MacMARCKS. A 42-kDa protein with the same molecular weight as MacMARCKS was identified in macrophage lysates by Western analysis using the antibody. Lipopolysaccharide- and PMA-activated macrophages from mice fed the trimyristin diet had significantly greater levels of MacMARCKS than LPS- and PMA-activated macrophages of mice fed the safflower oil-containing diet. The levels of MacMARCKS were also greater in lysates of LPS plus rIFNgamma-stimulated macrophages from mice fed the trimyristin diet and mice fed a diet containing a moderate level of myristic acid (12 g/100 g fatty acids) compared with the lysates of macrophages from mice fed the safflower oil diet. These results indicate that altering the level of myristic acid in the diet may alter the production of specific proteins that may be involved in macrophage activation.