Fifteen transplanted human kidneys with clinical and histological signs of chronic rejection were examined microscopically for cellular infiltration. Three normal kidneys were used as a reference. All infiltrating cells were classified and counted. The average number of cells per 10 microscopic fields was called the relative density of cellular infiltrates. Differences in the densities of different cell classes and changes in the cellular infiltration of the grafts were regarded as cellular expressions of the immune response. Summarizing our results, we conclude that: 1) the chronic immunological rejection of transplanted human kidneys is essentially caused by immunocompetent cells; 2) plasma cells develop in the graft itself; 3) the immunocompetent cell population tends to be purely and simply made up of plasma cells; 4) therefore, the true "effector cell" among the immunocompetent cells may be the plasma cell--at least in the chronic rejection of transplanted human kidneys. Cleaved lymphocytes were the most frequent of the infiltrating cells found in the transplants. However, their role is not yet clear.