During our isolation of biologically active substances from the spores of Ganoderma lucidum (Reishi Houshi, G. lucidum) guided by the inhibitory activity on HL-60 cell proliferation, NMR spectroscopic and mass spectrometric data indicate the substance contains a mixture of several long chain fatty acids. Hence, in this study, we have examined the inhibitory effects of an ethanolic extract of the spores of G. lucidum as the spore extract, on the proliferation of various human cancer cell lines by comparison with several authentic long chain fatty acids. Of the fatty acids we examined nonadecanoic acid (C19:0) showed the highest inhibitory activity for HL-60 cell proliferation with IC₅₀ values of 68±7 μM followed by heptadecanoic acid (C17:0, 120±23 μM), octa- (C18:0, 127±4 μM) and hexadecanoic acids (C16:0, 132±25 μM), respectively. The corresponding unsaturated fatty acids containing one double bond such as cis-10-nonadecenoic acid (C19:1), cis-9-octadecenoic acid (C18:1), cis-10-heptadecenoic acid (C17:1) and cis-9-hexadecenoic acid (C16:1) were less effective. The ethanolic extract of spores of G. lucidum were shown by annexin-V FITC/PI double staining to induce apoptosis of HL-60 cells in a similar way to cis-10-nonadecenoic acid (C19:1). These unsaturated fatty acids probably inhibit tumor necrosis factor production induced by lipopolysaccharide in a mouse macrophage preparation. Our results suggest the spores of G. lucidum contain 19-carbon fatty acids as one of the components for characteristics of its physiological effects.