Abstract
The properties of hematopoietic cells which serve as the precursors of cortical thymocytes in irradiated reconstituted mice have been described. These cells have been termed "prothymocytes." They are 10- to 15-mum diameter cells of low buoyant density. They are nonadherent to glass wool and more resistant to the lytic effects of steroids and gamma-irradiation than their progeny. They lack detectable amounts of the surface markers associated with either B or T cells but do bear at least two antigens recognized by antisera to mouse brain.