The behaviour of benzpyrene sarcoma, HeLa, Landschutz ascites, and Yoshida ascites sarcoma cells implanted into 16–18 h old primitive-streak stage chick embryos has been studied. Normal heterografts such as mammalian liver or pituitary were enveloped by abundant host mesoderm and thus practically isolated. This host mesodermal response (HMR) was not elicited to any significant degree by neoplastic cells. HeLa cells treated with chicken gamma globulin produced HMR with considerably enhanced frequency. Specificity was noticed in the interaction between the tumour cells and embryonic cells. The tumour cells showed mobility in the embryonic surroundings. HeLa and Land-schiitz cells appeared to direct pseudopodial activity specifically towards embryonic mesoderm. HeLa cells lost their mobility upon treatment with gamma globulin. Both normal and treated HeLa cells consistently formed an epithelium-like structure in the neighbourhood of the host epiblast. It has been suggested that such specificity of behaviour may be due to the amount of net negative charge present on the surfaces of the interacting cells. Yoshida ascites cells were found to be widely dispersed in the embryo, but they showed no specificity in their interaction except that they induced proliferation of the host epiblast cells in a small number of case