The adhesion of leukocytes to endothelial and other cell types is an essential part of the acute inflammatory response. One means by which adherence can be increased is by activation of the CD11/CD18 family of leukocyte glycoproteins. Chemotactic peptides, lipid mediators, phorbol esters and tumour necrosis factor are all able to increase the cell surface expression of one member of this family, CD11b/CD18 or Mac-1, by an unknown signal transduction mechanism. In this report, regulation of Mac-1 expression by C5a is shown to be independent of protein kinase C (PK-C) activation. The inhibitor of PK-C, H-7, has no effect on the action of C5a and only a slight effect on phorbol ester-induced up- and down-regulation of Mac-1, at a concentration that inhibits superoxide production in response to both factors by 40%. Inositol phospholipid hydrolysis, an important pathway leading to PK-C activation, and the transient increases in cytosolic Ca2+ associated with inositol phosphate production, are also shown to be not essential processes in C5a-stimulation of Mac-1 expression.