The ability of cord factor (trehalose-6, 6′-dimycolate) and a range of shorter carbon chain fatty acid trehalose diesters to suppress growth of an ascitic rat hepatoma has been examined and compared with that of whole, living BCG organisms. Aqueous suspensions of BCG, and cord factor in 0.4% arachis oil:Triton emulsion, injected intra-peritoneally, retarded growth of up to 105 ascites tumour cells. Trehalose-6, 6′-dibehenate was also tumour-suppressive, but only against lower challenge inocula (104 cells). Trehalose-6, 6′-dipalmitate and 6,6′-di-0–2-tetradecyl -3-hydroxyoctadecanoyl β,β trehalose (designated C76) were virtually ineffective and 6,6′ -di-0–2-eicosyl-3-hydroxy-tetracosanoyl β, β trehalose (designated C100) gave small and variable effects only against low challenge inocula. However, improved responses were seen with light mineral oil in place of arachis oil in the emulsions.