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Rabbit red blood cells (RaRBC, 3 × 107/ml PBS) were incubated with different amounts of purified human C3 at 37 for 30 mm and washed twice in PBS. Different amounts of normal human serum containing 2 mM Mg2+ and 5 mM EGTA were added to the C3-treated and control RaRBC. The extent of lysis was measured after a further incubation at 37 for 40 mm. Enhanced lysis was observed with C3-treated RaRBC as compared to control cells. The enhancing effect was dependent on the dose of C3 used for the treatment of RaRBC. Investigating the kinetics of lysis. the lag phase was observed to be significantly shorter with the C3-treated than with the control RaRBC. No enhancement was found when RaRBC were pretreated with preformed C3b fragment. KSCN-treated C3 (C3b-like C3), however, had a lysis- enhancing effect. These results suggest that non- covalently bound C3 molecules may have a role in the initiation and/or maintenance of the alternative path- way activation on activator cells. [ABSTRACT FROM AUTHOR]