The human leukocyte-function-associated antigen-1 (LFA-1) plays a key role in intercellular adhesion interactions of the immune response. A monoclonal antibody (mab), designated LDA-8, is described that recognizes LFA-1. In contrast to nearly all other anti-LFA-1 mabs, which inhibit cellular aggregation, LDA-8 induces cell-cell aggregation. The LDA-8 mab was generated by immunizing mice with membrane fragments from the Jurkat T-cell line. The LDA-8 mab stained peripheral blood mononuclear cells (PBMC), monocyte-depleted peripheral blood lymphocytes, purified monocytes, and a number of T and B tumor cell lines. The LDA-8 mab induced aggregation of PBMC from normal donors, as well as of cells from T-cell lines (MOLT4 and CEM). Control mabs directed against HLA class 1 or CD4 did not induce aggregation. Aggregation was concentration- and time-dependent. EDTA added to the cultures 1 hour prior to or together with the LDA-8 mab did not inhibit LDA-8-induced aggregate formation. Anti LFA-1 alpha-chain mab added to the cells 1 hour prior to LDA-8 mab, or together with the LDA-8 mab, also did not inhibit LDA-8-induced aggregation. In contrast, anti-LFA-1 beta-chain mab, added to the cells together with or 1 hour prior to the LDA-8 mab, significantly inhibited LDA-8-induced aggregate formation. The LDA-8 mab immunoprecipitated two polypeptide chains of 110 kDa and 160 kDa under non-reducing conditions and of 92 kDa and 162 kDa under reducing conditions, from cells of the MOLT-4 or CEM T-cell lines or phytohemagglutinin (PHA)-stimulated PBMC. The molecular mass of these polypeptides was identical to that of polypeptides immunoprecipitated by the anti-LFA-1 TS1.22 mab, suggesting that the LDA-8 mab and the anti-LFA-1 mab recognize the same molecule. This was confirmed by sequential immunoprecipitation. The LDA-8 mab recognizes a unique epitope on LFA-1 and induces cell aggregation that is blocked by mabs recognizing the beta-chain, but not the alpha-chain of the LDA-1 molecule.