We have made several T-T hybridomas which secrete soluble factors capable of suppressing an in vitro antibody response to nitrophenol (NP), but not other unrelated antigens. These factors bind specifically to NP, and express determinants coded for in the I-J region of the mouse major histocompatibility complex. No determinants that cross-react with the constant regions of mouse immunoglobulins are present on the factors. Three sub-clones originating from the same initial culture well of hybri...