The interaction between cloned amplifier T cells and stimulating cells has been probed with the use of a monoclonal antibody selected for its ability to inhibit stimulation of Mls- and la-reactive T cells. The antibody recognizes a public determinant on I-A- and I-E-coded molecules of a variety of murine H-2 haplotypes. The antibody inhibits the proliferation of the I-Ab-reactive T cell clone 48.1 and of the Mls-reactive T cell clones L2 and Fa 13. The antibody may be added as late as 3 hr after initiation of culture to cause inhibition of L2 proliferation and release of interleukin 2. A profound inhibition of primary and secondary mixed leukocyte responses also is observed. The results indicate that a large fraction of Mls reactive T cells require interaction with la antigens.