A wheat-based diet produced severe biotin deficiency symptoms appearing at the age of ten to fourteen days and becoming very severe in the third and fourth week (group 1). Biotin supplementation with 50 mug/kg (group 2) reduced the symptoms almost completely, but did not restore completely growth compared to chicks receiving the diet supplemented with 300 mug biotin/kg compared to chicks receiving the diet supplemented with 300 mug biotin/kg (group 3). The plasma level of biotin was about, or lower than, 100 ng/100 ml plasma in groups 1 and 2, indicating biotin deficiency. In group 3, plasma biotin was above 200 ng/100 ml. Liver biotin, after two weeks, was low in group 1 (less than 600 ng/g), medium in group 2 (1000 to 1500 ng/g) and in group 3 above 2000 ng/g. Plasma and liver biotin levels are found to be suitable parameters for diagnosis of subclinical biotin deficiency in chicks.