The CD19 antigen (95kDa) is expressed from the earliest stage of B progenitor development, on all peripheral B cells including germinal centre B cells, and all B cell lines and B cell leukaemia tested. T cell and monocytic cell lines are negative and the antigen is lost on B cell maturation to plasma cells. The antigen is a type I integral membrane glycoprotein whose in vitro inhibition will influence B cell activation and proliferation.