Since tumor-antitumor agent-host correlation must be considered in the treatment of neoplasms, the qualitative change of serum protein and hemolytic plaque forming cell (HPFC) production against sheep red blood cells (SRBC) in mice were carefully studied in part 1 to investigate the effect of a new antileukemic agent, Neocarzinostatin (NCS), on humoral immunity. Serum gamma globulin significantly decreased by the 4(th) day and returned to normal by the 12th day after a single injection of NCS. After 4 consecutive days of NCS administration gamma globulin value showed no change, but after 7 consecutive days of NCS 0.50mg/kg/day administration marked gamma globulin reduction was observed. Marked reduction of HPFC was observed in the -8(th) and -4(th) day groups of the single injection of NCS, but the -12(th), O, +2(nd) and +4(th) day groups showed only moderate reduction (Immunization by SRBC was done on 0 day.). After 4 consecutive days of NCS injection only 0.50mg/kg/day group showed HPFC reduction. But after 7 consecutive days of NCS administration 0.005mg/kg/day group showed moderate and 0.05mg/kg/day group showed marked reduction of HPFC. All mice in 0.50mg/kg/day group died. These data suggest that NCS suppresses the humoral immunity of mice to some extent and it affects almost all phases of the immune response but mainly the afferent phase. Furthermore, immunosuppressive effects of NCS are related to the daily dose and the number of consecutive days of NCS administration.