(-)-Indolactam-V, which has the partial structure of teleocidins A and B, and has tumor-promoting activity, is a good model for use in studies on the relation between structure and tumor-promoting activity, whereas (+)-indolactam-V has no tumor-promoting activity. In this work, five racemic indolactams differing only in their alkyl group at C-12 of (-)-indolactam-V were synthesized and tested for biological and biochemical activities related to tumor promotion. The activities tested were inductions of ornithine decarboxylase in mouse skin and human promyelocytic leukemia (HL-60) cell adhesion, inhibition of specific [3H]12-O-tetradecanoyl-phorbol-13-acetate binding to a mouse particulate fraction and activation of protein kinase C in vitro. The results showed that (+/-)-indolactam-L and (+/-)-indolactam-F had almost the same activities as (+/-)-indolactam-V, suggesting that (-)-indolactam-L and (-)-indolactam-F are new tumor promoters with as high potency as (-)-indolactam-V. (+/-)-Indolactam-t-L, which has a highly lipophilic group at C-12 of (-)-indolactam-V, showed the highest activities in the above tests. (-)-Indolactam-t-L might have stronger tumor-promoting activity than (-)-indolactam-V. Furthermore, the results with (-)-indolactam-t-L indicated the possibility of designing new tumor promoters with stronger activity than teleocidin.