Previous T cell receptor (TCR) sequence analysis of a panel of 23 H-2Kd restricted cytotoxic T lymphocyte (CTL) clones recognizing the decapeptide HLA-CW3 170-179 revealed a striking conservation of TCR structure, in that all clones examined used V beta 10 and J alpha pHDS58 segments. We show here that the primary response in vivo after intraperitoneal injection of DBA/2 mice with HLA-CW3 expressing transfectants of syngeneic P815 (H-2d) tumor cells is characterized by a dramatic expansion of CD8+ V beta 10+ CTL in the peritoneal cavity and draining (mesenteric) lymph node, as well as in peripheral blood. Additional analysis of TCR on HLA-CW3 immune populations by flow cytometry and polymerase chain reaction further indicates that the vast majority of responding CD8+ cells express restricted V alpha domains, a dominant J alpha segment (pHDS58), and a conserved CDR3 length for both alpha and beta chains. This novel system provides a unique opportunity to directly monitor an oligoclonal primary antigen specific immune response in vivo at the single cell level independently of functional assays.