All of the vanadate and vanadyl compounds tested in this study showed potent antibacterial activity against Streptococcus pneumoniae : the MIC (minimum inhibitory concentration) values of the vanadium compounds were 4-32 μg/ml, while the MIC values of tungstates and molybdates were 128->8000 μg/ml. Scanning electron microscopy showed the elongation of S. pneumoniae strains under low concentrations (<MIC) of vanadium compounds such as (tert-BuNH₃)₄[V₄O₁₂] (1), (tert-BuNH₃)₆[V₁₀O₂₈] (2), Na₃VO₄ (7) and VOSO₄ (8). The vanadium compounds non-selectively inhibited the incorporation of thymidine, uridine, leucine and glucose into the cells of S. pneumoniae and led to an efflux of potassium ions from the cells. It implies that the vanadium compounds interfere in or on the cell membrane with the transport of substrates and ions through the cell membrane, resulting in antibacterial activity against S. pneumoniae cells.