Granulocyte-macrophage colony stimulating factor (GM-CSF) has been shown to inhibit the growth and progression of murine melanoma cells in syngeneic C57BL/6 (H-2b) recipient animals. We now demonstrate that this effect is not specific to melanomas derived from a single strain of mice by examining the subcutaneous growth of K1735 murine melanoma cells (H-2k) transfected with GM-CSF in a syngeneic mouse model. Non-GM-CSF-secreting melanoma cells (parental K1735 and K1735 cells transfected with the GM-CSF gene in antisense orientation) generated tumours that reached a mean volume of 4000 mm3 30-40 days, with a mean survival of 40 days after tumour cell injection. In contrast, 90% of the mice injected with three different clones of GM-CSF-producing K1735 melanomas developed no measurable tumours and were healthy and tumour-free when followed for over 300 days post-inoculation. Additionally, mice injected with GM-CSF-secreting K1735 cells developed long-lasting immunity to the parental melanoma cell line challenge in vivo. A dense neutrophilic and lymphocytic inflammatory infiltrate as well as large numbers of dendritic cells were detected only at the inoculation sites of the GM-CSF-producing melanoma cells. Thus, these studies demonstrate for the first time that GM-CSF inhibits melanoma growth in a second genetically distinct MHC tumour-host model system and further support the use of GM-CSF in clinical trials in the treatment of advanced malignant melanoma in humans.