ParisThe effect of passively administered anti-2,4-dinitrophenyl (DNP) IgG, and IgG2 antibodies o n their own production has been studied.Guinea pigs were immunized with immune complexes emulsified in complete Freund's adjuvant. These complexes were made at 4 times equivalence with IgG, and/or IgC2 anti-DNP antibodies having the same affinity. Humoral IgGl and IgG, anti-DNP antibody contents were assayed at weekly intervals, and 4 and 10 days after a booster injection of antigen in saline given on the 12th week.Active anti-DNP IgC, antibody production is strongly inhibited by passive IgG2, IgG, and F(ab'), antibody fragments.Active anti-DNP IgG, antibody production of the same animals is inhibited by passive IgG2 and F(ab')2, t o an extent similar t o active IgC, antibody inhibition, but the response after boosting is normal. On the other hand, passive IgGl antibodies lead t o a delayed enhanced active IgG, antibody response.The effect of a 1 : 1 mixture of both passive antibody classes is intermediate t o what is observed with either class alone.These differential effects according t o the class ( i e . the Fc fragment) of the passive antibodies used show the importance of immune complexes in not only the quantitative, but also the qualitative, regulation of the immune response.