The mechanism of growth stimulation in allogeneic lymphocytes mixed in vitro was studied at the cell level by means of cytophotometric techniques. A pronounced increase in fluorescence intensity of fixed and acridine orange (AO) stained lymphocytes was observed as soon as after 1-3 hr in mixed culture. No increase in the amount of DNA took place during this time. The higher fluorescence intensity was due to an increased accessibility of AO binding sites in the deoxyribonucleoprotein (DNP) complex, most probably as a result of weakened bonds between the DNA and the protein moiety in the DNP complex. Similar DNP changes have been found in other systems of growth stimulation and may be one prerequisite for later induction of cellular synthetic processes. Increased AO binding only occurred when the lymphocyte donors were incompatible at the major histocompatibility locus (HL-A); there was no change in AO binding in cases of HL-A identity. The AO binding reaction probably reflects a specific recognition of HL-A antigens, whereas other antigenic discrepancies between the individuals do not seem to cause an analogous response.