Summary Several cohorts of cancerous mice were continuously dosed with 2,2′‐bipyridyl dibutyltin dichloride, 1,10‐phenanthroline dibutyltin and histidine dibutyltin; presented at 1 ppm and 10 ppm in drinking water. Tumour growth rates were significantly reduced. Tributyltin fluoride (TBTF) applied dermally, using dimethyl sulfoxide (DMSO) as the carrier, over the tumour site was ineffective. Variation in thymus and spleen weight between controls and tin‐exposed mice indicates a lymphatic involvement. Analysis of mouse tissue for total tin content showed high tin concentrations in the thymus and spleen.