The involvement of cytochrome-P450-dependent metabolism of small-molecular-weight compounds as a prerequisite for an optimal lymphocyte response to those compounds is discussed on the basis of studies with compound such as p-phenylenediamine, sulfamethoxazole, chloramphenicol and fragrances. Subsequently, data on cytochrome P450 activity and CYP gene expression in monocytes and MAC-6 cells for cytochromes P450 2E1 and 3A4 are presented.