The CH/pi interaction between the indole ring of indolactam-V (IL-V) and the hydrogen atom at position 4 of Pro-11 of the PKCdelta C1B domain was evaluated using the mutant peptide of the PKCdelta C1B domain, in which the CH/pi interaction was inhibited by substitution of the hydrogen atom with a fluorine atom. IL-V showed about a 10 times lower binding affinity to the mutant peptide compared to the wild-type peptide, suggesting that the CH/pi interaction could play a pivotal role in the binding of IL-V to the PKCdelta C1B domain. On the other hand, benzolactam-V8 (BL-V8), with the benzene ring instead of the indole ring of IL-V, might lack the CH/pi interaction. The low binding affinity of BL-V8 could be enhanced by the effective formation of the CH/pi interaction as exemplified by the synthesis of naphtholactam-V8 (NL-V8).