The aim of this study was to investigate the mechanisms by which interleukin‐10 (IL‐10) induces tumour growth in a mouse‐melanoma model. A B16‐melanoma cell line (B16‐0) was transfected with IL‐10 cDNA and three clones that secreted high (B16‐10), medium and low amounts of IL‐10 were selected. Cell proliferation and IL‐10 production were compared in vitro, and tumour growth, percentages of necrotic areas, tumour cells positive for proliferating cell nuclear antigen (PCNA), IL‐10 receptor (IL‐10R) and major histocompatibility complex type I (MHC‐I) and II (MHC‐II), as well as infiltration of macrophages, CD4⁺ and CD8⁺ lymphocytes and blood vessels were compared in vivo among IL‐10‐transfected and non‐transfected tumours. Proliferation and tumour growth were greater for IL‐10‐transfected than for non‐transfected cells (P < 0·001), and correlated with IL‐10 concentration (r ≥ 0·79, P < 0·006). Percentages of tumour cells positive for PCNA and IL‐10R were 4·4‐ and 16·7‐fold higher, respectively, in B16‐10 than in B16‐0 tumours (P < 0·001). Macrophage distribution changed from a diffuse pattern in non‐transfected (6·4 ± 1·7%) to a peripheral pattern in IL‐10‐transfected (3·8 ± 1·7%) tumours. The percentage of CD4⁺ lymphocytes was 7·6 times higher in B16‐10 than in B16‐0 tumours (P = 0·002). The expression of MHC‐I molecules was present in all B16‐0 tumour cells and completely negative in B16–10 tumour cells. In B16‐0 tumours, 89 ± 4% of the whole tumour area was necrotic, whereas tumours produced by B16‐10 cells showed only 4·3 ± 6% of necrotic areas. IL‐10‐transfected tumours had 17‐fold more blood vessels than non‐transfected tumours (61·8 ± 8% versus 3·5 ± 1·7% blood vessels/tumour; P < 0·001). All the effects induced by IL‐10 were prevented in mice treated with a neutralizing anti‐IL‐10 monoclonal antibody. These data indicate that IL‐10 could induce tumour growth in this B16‐melanoma model by stimulation of tumour‐cell proliferation, angiogenesis and immunosuppression.