PS-K, a protein-bound polysaccharide from a basidiomycetes, was found to suppress tumor growth after grafting of sarcoma-180 or Ehrlich tumor in ICR mice. In the present study, effect of PS-K on antibody-forming capacities was examined in tumor-bearing mice and normal controls. 1) PS-K did not enhance the capacities of normal mice to produce antibodies against sheep erythrocytes (SRBC), hamster erythrocytes (HRBC), and trinitrophenyl group (TNP). 2) The capacities of mice to produce IgG antibody against SRBC, IgM antibody aganist HRBC, and IgG antibody against TNP were depressed after grafting of sarcoma-180. Intraperitoneal injection of PS-K restored these capacities to the normal levels. 3) Oral as well as intraperitoneal administration of PS-K restored the capacity of the mice bearing sarcoma-180 to produce IgG antibody against SRBC. 4) The capacity to produce IgG antibody against SRBC was depressed after grafting of Ehrlick tumor and it recovered to the normal level after intraperitoneal injection of PS-K. These results showed that antibody-fforming capacity of mice was depressed after tumor grafing and recovered after administration of PS-K.