Chlorella has been used as a health food for the past 30 years in Japan and in other countries. Chlorella vulgaris has been shown to express various pharmacological effects in animals and humans. Hot water extracts of Chlorella vulgaris (CVE) are well known to be potent biological modifiers that have immune responses against tumors, bacteria, viruses, and native antigens such as casein. It was reported in 1995 that oral administration of CVE enhances resistance to Listeria monocytogenes by augmenting Listeria-specific cell-mediated immunity in both normal mice and mice with murine acquired immunodeficiency syndrome (MAIDS) caused by murine leukemia virus LP-BM5 MuLV. CVE is thought to augment Listeria-specific cell-mediated immunity in both normal and MAIDS mice, in association with increased numbers of CD4 + TCRαβ cells in the infected sites after L.monocytogenes infection. In our previous report in 1999, oral administration of CVE also inhibited IgE production against casein with an impaired Th2 response. In the field of environmental science, Chlorella vulgaris strain CK has shown to accelerate dioxin excretion in rats. The fecal excretions of dioxins were greater in rats fed Chlorella. These findings suggested that the administration of Chlorella may be useful in preventing gastrointestinal absorption and in promoting the excretion of dioxin already absorbed in tissues. As well, studies of γ-aminobutyric acid (GABA)-rich Chlorella and docosahexaenoic acid (DHA)-rich Chlorella between 2000 and 2002 were reviewed. γ-aminobutyric acid (GABA) is known to exert a hypotensive effect on blood pressure. A method has been established for producing large amounts of GABA in Chlorella vulgaris strain CK. It has been reported that the GABA-rich Chlorella showed a significant hypotensive effect in spontaneously hypertensive rats (SHR) and humans. Docosahexaenoic acid (DHA) has been shown to exert protective effects via numerous physiological actions. In order to produce DHA-rich Chlorella cells, Chlorella vulgaris strain CK was heterotrophycally grown in the pilot scale fermentor. The exogenous DHA was taken up and accumulated in Chlorella cells. Next, the effect of docosahexaenoic (DHA)-rich Chlorella vulgaris on hyperlipidemia has been studied in rats. An anti-hyperlipidemic effect was found in rats fed DHA-rich Chlorella vulgaris. Thus, the benefits of the Chlorella organism are shown to be very wide-ranging, and future studies can be expected regarding the nutritional fortification of Chlorella by adding functional substances to Chlorella cells.