Sublytic doses of complement desensitize cells and make them resistant to lytic complement doses. This process, named complement-induced protection, requires calcium ion influx, protein kinase C activation and protein synthesis. The involvement of the extracellular signal-regulated kinase, ERK, in cell desensitization by sublytic complement was examined in erythroleukaemia K562 cells and in COS-7 cells. As shown here, ERK is activated in K562 and COS-7 cells within 10 min of sublytic immune attack and then shows a decline and a second peak of activation at 20 min. C7- and C8-deficient human sera have a small effect on ERK activity. However, a significant increase in ERK activation is observed when C7 or C8, respectively, is added back to these sera. Complement-induced ERK activation was blocked in cells treated with GF109203X or Go6976, two selective PKC inhibitors, as well as by treatment with PD098059, an inhibitor of MEK1, the ERK kinase. PD098059 treatment also sensitized K562 cells to complement-mediated lysis and prevented complement-induced protection. COS-7 cells transfected with a dominant-negative MEK plasmid were incapable of undergoing the process of complement-induced protection. In conclusion, cell desensitization by sublytic doses of the complement membrane attack complex involves a signalling cascade that includes PKC-mediated ERK activation.