A cDNA clone was isolated from a carrot (Daucus carota L.) cDNA expression library using monoclonal antibody 21D7, which recognizes a nuclear antigen associated with cell division in plant cells. To show that the isolated cDNA encodes the 21D7 antigen, a polyclonal antiserum was raised against a recombinant fusion protein specified by the cDNA. Both the polyclonal antiserum and the monoclonal antibody 21D7 recognized the same plant protein on immunoblots, in immunoprecipitation experiments, and in peptide mapping. Analysis of the cDNA revealed that the deduced amino acid sequence has 45% identity to the predicted sequence of the mouse transplantation antigen P91A from mutant tumor cells that is responsible for the immune rejection of the corresponding cell clone in a syngeneic mouse. The expression of the plant cDNA at the mRNA level was highly correlated with cell proliferation. In suspension cultures of Catharanthus roseus (L.) G Don. cells, the highest level of expression was observed during the midlogarithmic phase of growth. When auxin was added to stimulate cell division of auxin-starved cells arrested in the G1 phase, transcription was immediately enhanced, and the level of expression remained high throughout the G1 and S phases and dropped dramatically at the end of DNA replication.