Lymphocytes and mononuclear phagocytes are essential to host defense, yet little is known about their metabolic requirements. To determine the involvement of carnitine in the intermediary metabolism of these cells, the amounts of free carnitine and individual acylcarnitines were determined for human peripheral blood lymphocytes and mononuclear phagocytes. Lymphocytes from healthy young adults contain 79 +/- 6 and 56 +/- 5 nmol/10(9) cells total and free carnitine, respectively, showing a 29% acylation. When expressed per mg cell protein, they contain 3.41 +/- 0.54 nmol total and 2.44 +/- 0.40 nmol free carnitine. By comparison, phagocytes contained approximately 4-fold more total carnitine per cell (301 nmol/10(9) cells) and had a much higher level of acylation (69%). Acetylcarnitine was the predominant acylcarnitine in both lymphocytes (15.4 nmol/10(9) cells) and phagocytes (98.3 nmol/10(9) cells) accounting for 72-73% of total acylcarnitines. Long-chain acylcarnitines constituted 3-4% of total acylcarnitines. These data suggest that carnitine is involved in the metabolism of both short and long-chain acyl-CoAs within lymphocytes and phagocytes, but its specific roles in these two cell types have not been determined.