Mixed lymphocyte reactions (MLR) with cells from DLA-identical dog sibs are nonresponsive either in primary or in secondary interactions after in vitro priming. However, lymphocytes from animals sensitized in vivo against DLA-identical sibs can show increased proliferative response to their immunizing donor. The kinetics of this reaction are similar to MLR across the DLA-barrier, although its intensity is less important. This proliferation may reflect minor histocompatibility antigenic differences.