Monocytes, dendritic cells, macrophages, and T-helper/regulatory T-cells all express the 55 kD glycoproteinknown as CD4. The immunophenotyping oflymphoproliferative diseases involves the use of anti-CD4.The T-helper/regulatory cell subgroup is the source of mostperipheral T-cell lymphomas, resulting in CD4+ CD8-mature T-cell neoplasms. Like other T-cell antigens, CD4 canbe aberrantly expressed in neoplastic T-cells. To identifytumours with CD4 aberrant expression, a panel of markersmust be applied during the examination of such tumours.