In the purification of elaiophylin from a culture of Streptomyces hygroscopicus MCY -846, mono- and di-methyl-elaiophylin were obtained through a O-methylation of the hemiketal hydroxy group of elaiophylin. All the three elaiophylins showed cytotoxicity against several human tumor cell lines and murine cell lines. Elaiophylin and monomethyl-elaiophylin also showed antimicrobial activities against gram-positive bacteria and potent inhibitory effects on the activation of B cells by lipopolysaccharide as well as on the proliferation of mouse splenic lymphocytes stimulated by mitogens but dimethyl-elaiophylin did not. This result indicates that elaiophylin and monomethyl-elaiophylin would be strong immunosuppressants. Furthermore this result revealed an interesting structure-activity relationship suggesting that the lack of symmetry and/or the free OH group at C-11 of elaiophylin might be important in conferring biological activities.