One of six transplantable ascites tumors of BALB/c mice was found to become periodically resistant to cytotoxic T lymphocytes (CTL). About 12 days after LPC-1, a myeloma tumor, was transplanted it became resistant to lysis by allogenic CTL (anti-H-2d) and by CTL directed to trinitrophenyl groups or minor histocompatibility antigens. Susceptibility to lysis by all of these CTL was regained within 2 to 4 days after transfer of resistant cells to a fresh BALB/c host. These changes were recurrent: in each transplantation cycle the early LPC-1 cells were susceptible and the late cells were resistant to CTL. Analyses with antisera (B10 anti-B10.D2) showed that the serologically recognized products of the H-2d haplotype were reduced about 10-fold on the LPC-1 cells that were resistant to CTL.