Abstract 1. The individual and combined effects of aflatoxin B1 (AFB1) and DL‐gizzerosine (GIZZ) in growing broiler chickens were investigated in a 7‐d fully randomised trial consisting of 7 dietary treatments (1 and 4 mg GIZZ/kg, 0.66 and 3 mgAFB1/kg, the combinations 1 mg GIZZ/kg/0.66 mg AFB1/kg and 4 mg GIZZ/3 mg AFB1/kg, and a control basal diet without GIZZ or AFB1). 2. Dietary GIZZ at 1 mg/kg caused moderate gizzard erosions without affecting performance, whereas 4 mg/kg GIZZ induced severe erosion, ulceration and perforation of the gizzard, and significantly affected performance. 3. Dietary AFB1 at 0.66 or 3 mg/kg did not induce lesions in the gizzard or cause any adverse effects on chick performance. 4. No apparent toxicologic interaction was observed between AFB1 and GIZZ when they were given simultaneously at relatively low dietary concentrations (0.66 mg/kg AFB1 + 1 mg/kg GIZZ). However, at higher dietary concentrations (3 mg/kg AFB1 + 4 mg/kg GIZZ), AFB1 potentiated the lethality of GIZZ. 5. The combination 3 mg/kg AFB1/4 mg/kg GIZZ was more toxic than the same concentrations of single toxins and may pose a greater economic threat to the poultry producer than either toxin individually.