AIMS: Copaifera multijuga Hayne oleoresin is commonly used in traditional medicine owing to its anti-inflammatory, antiseptic, antitumor, and antibacterial properties. However, little is known about the effect of the compounds from the bark of this plant. In this study, the immunomodulatory effect of the ethanolic extract of C. multijuga bark via natural killer activity of non-adherent spleen cells of Ehrlich tumor-bearing mice was evaluated.METHODS: Male Swiss mice were inoculated subcutaneously with 1×106 Ehrlich tumor cells (Ehrlich and Ehrlich/C. multijuga group) or phosphate buffered saline solution (control group and C. multijuga group) and treated orally daily with C. multijuga extract (200 mg kg-1, 0.1 mL per mouse, for the Ehrlich/C. multijuga and C. multijuga groups) or phosphate buffered saline solution (control group and Ehrlich group). The four experimental groups consisted in eight mice each and were organized in two sets, one treated for seven days and another treated for 14 days, totalizing 64 mice throughout the experiment. Twenty-four hours after the last oral administration, the mice were euthanized and the spleen tissue was isolated to prepare a non-adherent spleen cell suspension and to evaluate natural killer activity. Data are presented as the cell lysis percentage of Yac.1 target cells by non-adherent spleen cells.RESULTS: Treatment for seven days increased natural killer activity in the Ehrlich/C. multijuga group (21.20±8.89, p<0.05) compared to the control group (3.14±2.71, p<0.05); however, this effect was not maintained in the groups treated for 14 days (Control: 6.02±6.98, Ehrlich: 4.82±7.72, C. multijuga: 2.07±2.10, Ehrlich/C. multijuga: 2.01±1.63, p>0.05).CONCLUSIONS: Treatment for seven days with an ethanolic extract of C. multijuga bark enhanced the natural killer activity of non-adherent spleen cells from Ehrlich tumor-bearing mice.