LFA-1 (CD11a) and Mac-1 (CD11b) are two major β2 integrins expressed on neutrophils. They mediate neutrophil adhesion and transmigration through endothelium and extracellular matrix. To investigate the specific contributions of Mac-1 and LFA-1 to neutrophil functions in vivo, we developed CD lib-deficient mice. A replacement construct was developed from 129 sv genomic clones which deleted exon 4. Homozygous mice for the Mac-1 mutation (Mac-1-/-) were confirmed completely deficient in Mac-1 by RNA protection assays and flow cytometry with mAb M1/70 (anti-CD11b). Isolated neutrophils from Mac-1-/- mice completely lacked several Mac-1-dependent functions in vitro: They did not (1) adhere to KLH-coated glass; (2) develop oxidative burst when stimulated by serum-activated zymosan particles; and (3) exhibit homotypic aggregation when stimulated by PMA. LFA-1 expression in Mac-1-/- mice was not different from that in Mac-1+/+ siblings, and functioned normally as shown by homotypic aggregation of purified T cells, previously shown to be LFA-1- and ICAM-I-dependent in Mac-P +. When challenged by thioglycolate i.p. for 4 hours, Mac-1-/- mice had similar levels of neutrophil accumulation in the peritoneal cavity with no significant difference from Mac-1+/+ mice: Mac-1+/+ 15.3±5.3x106, Mac-1' I9.6±11.5×106. Systemic treatment with mAb KBA (anti-LFA-1) caused a maximum of 80% inhibition of neutrophil accumulation in Mac-1-/- mice and 60% inhibition in Mac-1+/+ mice. These data demonstrated that in the absence of Mac-1, LFA-1 is sufficient to mediate significant neutrophil transmigration. LFA-1 may play a more important role in neutrophil trafficking than Mac-1.