Abstract Experimental allergic encephalomyelitis (EAE) was induced in obese strain (OS) chickens with spontaneous autoimmune thyroiditis (SAT). The effects of surgical bursectomy (BE) in ovo on the 18th day of incubation and of thymectomy (TE) on the day of hatching on the development of these two diseases were studied within the same animal. EAE was elicited in 6- and 11-week-old unoperated, thymectomized or bursectomized OS chickens by a single injection of a saline extract of bovine spinal cord and complete Freund's adjuvant, with Pertussis vaccine as a coadjuvant. Unimmunized birds and chickens injected with bovine liver extract served as controls. Neonatal TE led to an increase in the severity of SAT irrespective of the consecutive immunization, but resulted in a decrease of EAE. BE in ovo entailed a significant decrease of SAT, but also a marked suppression of EAE. Thus the bursa dependency of SAT in OS chickens has been confirmed, while both bursa-derived and thymus-derived lymphoid cells appear to be involved in the development of EAE in this strain.