Alpha-tocopherol but not 2-carboxy-2,5,7,8-tetramethyl-6-chromanol (trolox or CTMC) and 2,2,5,7,8 pentamethyl-6-hydroxy chromane (PMC), derivatives ofα-tocopherol, inhibited the superoxide (O2−) generation of rat peritoneal neutrophils (RPMN) induced by phorbol 12-myristate 13-acetate (PMA). ID50for neutrophils obtained from the peritoneal cavity of rat and guinea pig was about 1μM. This concentration, however, was much lower than that for the inhibition of PMA-activated phospholipid-dependent protein kinase (PKC) (ID50= 30μM). Theα-tocopherol sensitive O2−generation was also observed in neutrophils induced by dioctanoylglycerol (diC8) and calcium ionophore A23187 but not by formylmethionyl-leucyl-phenylalanine (FMLP), opsonized zymosan (OZ) and sodium dodecyl sulfate (SDS). The pattern of inhibition byα-tocopherol was quite similar to that of staurosporine, a specific inhibitor of PKC. Theα-tocopherol content of RPMN was 12 ng/106cells and a linear increase to 200 ng/106cells by addition ofα-tocopherol to the cell suspension corresponded with an increased inhibition of O2−generation. These results indicate that both the chemical structure and the content ofα-tocopherol might be important factors in O2−generation by neutrophils.