During HIV-I infection, cytotoxic T cell (CTL) responses exert strong selective pressure on the replicating virus population. Here we report evidence for T cell activity against the drug resistant K103N region of viral reverse transcriptase in three HIV-1 infected patients exposed to NNRTI antiretroviral drugs. We further characterize the response in one patient by ELISPOT analysis. A nine amino acid peptide incorporating 103N was recognized by patient T cells whereas the wild type was not. The RT K103N mutation is selected by the NNRTI class of HIV drugs. We hypothesize that, in certain individuals, CTL responses against 103N-containing epitopes may protect against NNRTI drug resistance. Characterizing such responses in the context of HLA subtypes could lead to tailored HIV drug therapy or to the design of therapeutic vaccines.