C3-independent immune haemolysis was studied using EAC14oxy2 cells and purified C5, C6, C7, C8 and C9. We have found that EAC14oxy2 cells were lysed by C5-C9 and that haemolysis occurs, even after pretreatment of the cells and the C5-C9 preparation with anti-C3. This indicates that EAC14oxy2 can be lysed by C5-C9 without any participation of C3. In contrast, EAC1 and EAC14 cells are not lysed by C5-C9, suggesting that our C5-C9 preparation lacks activated complement components, such as C3bBb, C5b6 or C(56)a. Based on our study of the haemolysis of EAC14oxy2, we have determined that: (i) EAC14oxy2 cells are lysed by a preparation of C5, C6, C7, C8 and C9, but no lysis occurs when any one of these complement is absent, (ii) for significant haemolysis of EAC14oxy2, a higher concentration of C5 is necessary as compared to the C5 requirement when haemolysis occurs in the presence of C3, (iii) the degree of haemolysis is linearly related to the concentration of C5 and does not reach a plateau, despite the addition of as much as 3,200 U of C5, and (iv) the degree of haemolysis is linearly related to the concentration of cell bound C42. These observations suggest that, in the absence of C3, the C3 convertase C42 can activate C5 directly, resulting in the formation of the membrane attack complex, C5b-9.