Eight murine anti-TNF alpha monoclonal antibodies (mAb) were produced after immunization of BALB/c mice with rhTNF alpha. Six of these mAbs were able to neutralize cytotoxic activity of TNF alpha against L929 cells. Two other mAbs had no neutralizing effect. Epitope mapping studies were performed by ELISA and by using a BIAcore system (Pharmacia). The described mAbs were allowed to define 4 different epitopes on TNF alpha. Three of them were involved in the binding of TNF alpha with its receptor (cytotoxic neutralization of TNF alpha). Another epitope was defined by non-neutralizing mAbs.