Tacrolimus is the new name for the Japanese immunosuppressant drug FK506. It blocks interleukin 2 transcription thereby inhibiting the CD4 Th-1 lymphocyte response to foreign antigens. It also selectively inhibits cytokine secretion of IL3, IL4 and interferon-gamma. It is metabolised by the cytochrome P-450 enzyme system. The results of two trials indicate that it is similar to cyclosporin in efficacy and toxicity.