To investigate the effect of hemolymph of silkworm larvae on the multiplication of Bombyx mori nuclear polyhedrosis virus (BmNPV) in BmN-4 cells, BmN-4 cells were infected with BmNPV, which were sequentially Heated with the hemolymph exracted from B. mori larvae. When the culture media TC-100 containing 3% fetal bovine serum was mixed with 10% hemolymph heated at 65$^{circ}C$ for 30 minutes, the released polyhedra by multiplication of BmNPV in BmN-4 cells were increased more than those of non-treated. However, multiplication of BmNPV in BmN-4 cells treated with non-heated hemolymph was not effective, since non-heated hemolymph was toxic for the cell growth. The result of plaque assay showed that plaque forming units in BmN-4 cells treated with heated hemolymph are significantly increased, suggesting that efficiency of multiplication of BmNPV in BmN-4 cells is due to increase not of cell growth but of infectivity of BmNPV.