1. The embryotoxicity of mixed aflatoxins (AF) and aflatoxin B1 (AFB1) were evaluated by a modified chick embryotoxicity screening test (CHEST). Adverse effects on the early embryonic development of thymus and bursa of Fabricius were also investigated by light microscopy. AF consisted of 83.06% AFB1, 12.98% AFB2, 2.84% AFG1 and 1.12% AFG2. 2. A total of 448 fertilised laying hens' eggs were used. AF and AFB1 were injected into the eggs at doses of 10, 100 and 1000 ng/egg. Embryonic developmental stages were evaluated according to the Hamburger-Hamilton scale (HH-scale). 3. The results showed that AFB1 given at 10 ng/egg had a significantly (P<0.05) greater embryotoxic effect than AF given at a similar dose. The higher doses of both AF and AFB1 caused higher embryonic mortality and also an increase in early deaths. 4. In the groups receiving 100 ng/egg AF and AFB1 an abnormal development was seen, with a protruded central region, corresponding to the area pellucida of the blastoderm. No other developmental abnormality attributable to AF or AFB1 was found. © 2000, British Poultry Science Ltd.