Gamma-delta-T cells represent a new lymphocyte subset without a definitive functional assignment. Although in many ways similar to alpha-beta-T lymphocytes, they are clearly distinguished by their expression of a different set of T cell receptor genes, a different distribution in normal tissues, and perhaps also different ligand specificities. Because gamma-delta-T cells appear to be involved in a variety of human diseases, the determination of their biological role has become an important challenge for immunologists and researchers in related areas.