1. Addition of supplemental choline to a biotin-deficient diet decreased the biotin status of chicks and increased mortality from fatty liver and kidney syndrome (FLKS). 2. Mortality was also increased by dietary supplementation with a mixture of other B-vitamins, excluding biotin, and was highest when the choline and B-vitamin supplements were combined. 3. The occurrence of sudden death syndrome (SDS) was unaffected by dietary biotin concentration. 4. A previously unreported condition was observed in which birds died showing post-mortem signs characteristic of both FLKS and SDS and whose occurrence was related to the biotin status of the chicks.