The specificity of in vitro induced human influenza-immune cytotoxic effector cells was analyzed with respect to recognition of HLA-A and -B-linked gene products. The influenza-immune cytotoxic activity observed on panels of virus-infected targets demonstrated that virus-immune effectors preferentially lyse targets with which they share HLA-A or -B specificities. Virus-immune effectors from certain donors recognized virus in conjunction with some, but not all, of their self HLA-A and -B antigens. Among donors who share a given HLA antigen (such as A2 or B7), there are differences in the ability of their virus-immune T cells to recognize the shared antigen. Virus-infected target cells from HLA-A2 or -B7 “nonresponder” donors could be lysed by virus-immune T cells obtained from other donors who shared only the HLA-A2 or -B7 antigen with these target cells. These observations suggest that the absence of cytotoxic T cell responses by some donors to influenza virus in conjunction with HLA-A2 or -B7 is not due to control by the structural genes that code for these HLA antigens, but rather may result from control by regulatory genes that act at the level of the responder and/or stimulator cell. The results are discussed in the context of Ir gene regulation of human T cell responses.