The leukocyte-restricted 92 (CD18) integrlns mediate cell adhesion in a variety of events essential for normal immune function. Despite extensive research in this field, only three members of this lntegrin subfamily have been descrfbed: CDllaKD18(LFA-l), CDllbl CD18 (Mac-l), and CDllclCD18(pl50, 95). We have identified a cDNA encoding a fourth a chain, ad, that associates with CD19 The ad subunit is more closely related to CDllb and CDllc than to CDlla. This integrfn is expressed at moderate levels on myelomonocytlc cell lines and subsets of peripheral blood leukocytes, and more strongly on tissue-compartmentalized cells such as foam cells, specialized macrophages found In aortlc fatty streaks that may develop Into atherosclerotic lesions. The ad/CD18 molecule exhibits preferential recognition of ICAM-over ICAM-1.