Following the characterization of tremerogen A-10, a novel lipopeptidal hormone for the induction of conjugation tube formation in the a type cell (IFO 9313) of the yeast Tremella mesenterica, another peptidal hormone (tremerogen a-13) that induces conjugation tube formation in the A type cell (IFO 9310) was recently isolated by Yoshida et al., and its structure was determined as H-Glu-Gly-Gly-Gly-Asn-Arg-Gly-Asp-Pro-Ser-Gly-Val-Cys (S-farnesyl)-OH by Sakagami et al., with the isoprenoid moiety at the carboxyl terminal cysteine being common to the sex hormone of Tremella mesenterica. To confirm the proposed structure, the farnesyl tridecapeptide was synthesized, and thin layer chromatograms of the synthetic product were identical with those of natural purified tremerogen a-13; the thermolytic peptide maps from the synthetic and natural products were also identical, and their biological activities were the same. Some analogs of tremerogen a-13 were then prepared to clarify the structure-activity relations of the carboxyl terminal part. The biological activity and TLC Rf values of tremerogen a-13 and its analogs showed that: 1) the C-terminal blocked analog (tremerogen a-13 amide) had higher potency than tremerogen a-13; 2) the farnesyl group was most favorable for high potency; 3) even the SH-peptide without a lipophilic C-terminal side chain showed slight activity. These results are interesting because for tremerogen A-10, a blocked C-terminus and lipophilic side chain are essential for activity, and derivatives with more lipophilic side chains (tetra- or pentaprenyl) than farnesyl at the C-terminal cysteine had higher potency than tremerogen A-10.