Mitogen-induced blast transformation of human peripheral blood lymphocytes was studied using a microplate culture technique. Cells and plasmas from normal individuals were interchanged to that sources of variation in the degree of transformation could be evaluated. Also evaluated was the normal range of plasma effects. It was found that the cells of different individuals vary considerably in their responses to mitogens in a given plasma, but the hierarchy of responses is fairly consistent from plasma to plasma. Furthermore, plasmas vary from donor to donor, but also settle into a fairly consistent hierarchy regardless of the cells employed. Finally, new hierarchies are established when materials from the same donors are collected after an interval of a week. It was shown that special, nonrandom interactions may occur which are not attributable simply to the average response of a given cell and the average response to the plasma used. The range of variation to be expected in a normal system, established here, should permit comparison of plasma effects in a more interpretable framework than previously was possible.