Lymphoid cell interactions in the immune response, both in vivo and in vitro, have been the subject of much interest recently (1, 2). In this regard, substances have been found in supernatants of cell cultures which can affect the response of other lymphoid cells (3, 4). The following report describes a new effect wherein mouse thymocytes are highly stimulated by a combination of nonspecific stimulants and substances released by human leucocytes. Five × 106 thymus or spleen cells from male CBA/H or CBA/J mice, aged 6 to 12 weeks, were cultured in duplicate in plastic tubes (Falcon #2001) in 1.0 ml MEM-S (Microbiological Associates, Bethesda, Md., with added antibiotics and l-glutamine), supplemented with 8% normal human serum. These cultures were stimulated by the following, added singly or in combination: a) 1.0 µl phytohemagglutinin-P (PHA, Difco Laboratories, Detroit, Mich.); b) different concentrations of human white blood cells (obtained by gravitational sedimentation of the erythrocytes); c) supernatants (SUP) from cultures of human leucocytes.