The in vitro activity of influenza-specific cytotoxic T cells can be inhibited by incubation of the target cells with monoclonal anti-influenza antibodies. Hybridoma antibodies that bind to the virus HA inhibit the cytotoxic activity of TDL for the virus-infected target by as much as 80%, whereas these same antibodies never reduce splenic T cell function by more than 40%. This reflects the fact that TDL from anti-influenza strain A/WSN/33 (HON1) are highly subtype-specific, whereas splenic effector cells from the same mice are cross-reactive for target cells infected with heterologous influenza A viruses. These findings are discussed in the light of previous failures to block virus-immune T cell effector function with heterogeneous antisera produced in vivo, and are considered to favor the idea that at least some of the "virus-immune" T cells are indeed recognizing viral antigens.