A Sephadex G-10 column coated with antigen-antibody complexes and complement 
retains complement receptor-bearing (CR+) mouse spleen cells. The effluent is 
rich in thymus-derived cells (T cells), and contains bone marrow-derived cells 
(B cells) which carry surface immunoglobulin (Ig), Ir-associated antigen (Ia), 
and Fc receptors, but no complement receptors (CR-). Although both 
unfractionated and CR- B cell populations are capable of producing antibody to 
red cell antigens, they differ in their requirements for the initiation of the 
response. Unfractionated B cells cooperate with primed as well as unprimed 
helper T cells; macrophages are required for this cooperation but can be 
replaced by 2-mercaptoethanol. CR- B cells cooperate with primed but not with 
unprimed T cells provided macrophages are added to cultures. After addition of 
culture supernatant from BCG-activated macrophages CR- B cells cooperate with 
both unprimed and primed T helper cells.