Thymoquinone (Tq) has been reported to potentiate the in vitro growth-inhibitory activity of some antibiotics especially against Staphylococcus aureus. However, it has been shown that Tq vapors can affect the results of susceptibility testing by standard broth microdilution method. Therefore, we made a comparative experiment with and without ethylene vinyl acetate cap mats (EVA capmatä) on microplates. The results showed significant differences in the minimum inhibitory concentration values and proved this capmat as an effective vapor barrier. Therefore further experiments focused on the in vitro anti-staphylococcal combinatory effect of Tq with oxacillin, penicillin, and tetracycline against various S. aureus strains have been performed by checkerboard method using EVA capmatä. The combined effect was evaluated according to the sum of fractional inhibitory concentrations (RFIC). Synergy was obtained for combination with oxacillin against 3