Oral administration of a hot water extract of Chlorella vulgaris(CVE)(20mg/mouse, 10 consecutive days) augmented the resistance against an i.p. infection with Listeria monocytogenes in mice. The numbers of bacteria in a CVE-administered group were significantly lower in the peritoneal cavity or spleen than those in a control group. FCM analysis revealed that gamma delta +Thy1.2+ cells in the nonadherent PEC from CVE-administered mice increased more prominently in number at the early stage on day 3 or on day 5 after infection as compared with those in control mice. The increment of gamma delta +Thy1.2+ T cells was also evident in spleen in CVE-administered mice at this stage after infection. The proportion of TCR alpha beta +Thy1.2+ T cells in the nonadherent PEC of a control group increased from 13% on day 0 to 49% at the late stage on day 10 after infection, whereas the proportion of TCR alpha beta +Thy1.2+ T cells in the nonadherent PEC in CVE-administered mice increased to 64% on this stage after infection in association with augmentation of DTH response to Listeria. These results suggest that CVE-administration effectively augment cell-mediated immunity against Listeria through the increment of gamma delta + T cells in the early phase and the increment of alpha beta + T cells in the late phase after listerial infection.