Production of granulocyte-macrophage colony-stimulating factor(s) (GM-CSF) by human tumours was investigated using heterotransplantation of a number of different tumours in nude mice. An increase in granulocyte numbers (> 20,000/mm3) in the peripheral blood of nude mice accompanied the growth of 9 of the 25 transplanted tumours. GM-CSF activity tested against normal human marrow cells was relatively high in 6 of these 9 tumours. Moreover there was either weak activity or none at all in 14 of the 16 tumours that failed to cause a definite granulocytosis. The correlation between granulocytosis and GM-CSF activity was 0.36, which was statistically significant (P < 0.01). These findings indicate that the transplantation of human tumours into nude mice can provide a useful tool for detection and characterization of granulopoietic factors derived from the tumours.