The integrin family consists of a series of related alpha beta heterodimers (subunits of 95,000-200,000 Mw) involved in a variety of cell-matrix and cell-cell adhesion functions. Leukocyte adhesion has biological importance in numerous processes involving host defense. The CD11/CD18 integrins are differentiated antigens which play a critical role in this mechanism. CD11a/CD18 are apparent on early progenitors of all myeloid and erythroid cells. CD11b/CD18 and CD11c/CD18 are more restricted antigens normally expressed on monocytes, macrophages, PMN and natural killer cells. Activated granulocytes and monocytes express far more CD11b/CD18 than the other two antigens: 6 to 8 x 10 (5) CD11b/CD18 molecules appear on maximally activated granulocytes. These integrins and in particular the beta 2 subunit are lacking in a genetic disease. On the other hand, they are fundamental in numerous physiological processes and in various hematological and cardiovascular diseases. The biochemical characterization and behavior of the CD11/CD18 complex in various clinical conditions are the subject of this review.