The effects of the histidine-containing dipeptide carnosine and of histidine on the activity of 15-lipoxygenase from rabbit reticulocytes are studied. It is shown that low concentrations of carnosine (0.5-4 mM) activate oxidation of arachidonic acid by lipoxygenase, while high concentrations (>4 mM) inhibit the enzyme activity. Histidine elicits no activatory effect, although its inhibitory activity is comparable to that of carnosine.