Using in situ hybridization, we analysed the immunoglobulin repertoire expressed by the B cells present in myasthenia gravis thymuses from four patients. B cells, mostly in activated state, were clustered in germinal centres, in which multiple isotypes were identified. A majority of cells expressed IgG as compared with IgM, with a roughly similar contribution of kappa and lambda chains. Hybridization with the six VH and the 4 VK human family probes was observed in serial sections, providing additional evidence that individual germinal centres were polyclonal. The thymic B cell repertoire closely reflected the VH and the VK family usage of normal peripheral blood lymphocytes with the preferential utilization of VH3, VK1 and VK3.