Experiments were carried out at Uhříněves, Czech Republic, in 1995 and 1996 to evaluate the effect of salinomycin, monensin, lasalocid and maduramicin on in vitro rabbit caecal fermentation. In cultures supplied with starch, hemicellulose and pectin, all ionophores in the range of concentrations tested (1–10 μg/ml) stimulated methanogenesis. Ionophores at 5 μg/ml increased the production of methane by 26–35% and decreased the production of acetate (P<0·05). Significantly more acetate was produced in mixotrophic acetogenic cultures of caecal micro-organisms with glucose (2 mg/ml) under hydrogen atmosphere than under nitrogen. The H2-dependent formation of acetate was blocked in cultures with ionophores. The stimulation of methanogenesis by salinomycin, monensin, lasalocid and maduramicin can thus be related to the inhibition of acetogenesis, which is an alternative method of H2 utilization in the rabbit caecum.