When a 12-y-old girl suffering from isovaleric acidemia was treated with L-carnitine, there was a considerable increase in her blood and urine concentration of isovalerylcarnitine. When later the patient received an infusion of glycine in place of carnitine, isovalerylcarnitine reverted toward the low levels found in a normal subject. At the end of either treatment, erythrocyte calpain was measured and found to be decreased after carnitine therapy (140 versus 96 U/mg Hb with glycine or carnitine, respectively). Because we have previously shown that the activity of calpain isolated from erythrocytes was markedly modified by isovalerylcarnitine, the present results might be seen as the consequence of the chronic exposure of the patient's red blood cells to high levels of isovalerylcarnitine. The lowered calpain activity was also proved by an increase in erythrocyte band 3 phosphorylation together with an increased erythrocyte fragility after calcium loading in the presence of the ionophore A-23187. Calpastatin, the natural inhibitor of calpain, was only slightly modified.