The antifungal activity of synthetic endothiopeptdides, i.e. Nva(psi)[CSNH]-FMDP and Lys(psi)[CSNH]-Nva(psi)[CSNH]-FMDP was studied in medium containing blood serum, against selected Candida strains; Candida albicans Gu4 (fluconazole sensitive), C. albicans Gu5 (fluconazole resistant), C. albicans ATCC 10231, Candida krusei DSM 6128 and Candida parapsilosis DSM 5784. Although thiopeptide bonds in the tested peptides increased their stability in blood serum, their antifungal activity, however, drastically decreased in comparison with the peptides containing non-modified peptide bonds. Moreover, the inhibitory activity towards glucosamine-6-phosphate synthase of thionated synthetic analogue of FMDP was performed. The thiopeptdide bond also influenced its inhibitory properties against enzyme from C. albicans.