BAIN et al.^1,2 have shown that when human peripheral blood leucocytes from two individuals are mixed and cultured, large immature cells appear and the incorporation of tritiated thymidine into DNA is stimulated. It has been suggested that variations in this response may reflect differences in the number of histocompatibility antigens shared by the donors of the cells^1–3. It has also been shown that transplantation antigens can be detected in the medium from tissue cultures of rabbit⁴ and dog⁵ spleen cells. We, therefore, decided to determine whether factors released into the medium from cultured peripheral blood leucocytes might stimulate tritiated thymidine uptake (radioactive content) in cultures of leucocytes from another individual.