This is the first report about adverse inhibitory effect of V-Guard and E-Guard gasoline additives against soil beneficial Streptomyces. V-Guard and E-Guard gasoline additives are anti valve seat recession agents used in unleaded gas for old car engines in Iran. They contaminate soil and groundwater by fuel leaks and spills. Streptomyces are of the major contributors to the biological buffering of soil environment by exerting antagonistic activity against wide range of bacteria and fungi. In order to elucidate antimicrobial activity of these additives, they were tested against fifteen soil isolates of Streptomyces and two plant pathogens including Erwinia carotovora and Fusarium solani. The additives did not reveal any growth inhibitory activity against E. carotovora and F. solani, but showed strong inhibitory effect against Streptomyces isolates. The Minimum inhibitory concentration (MIC) against Streptomyces isolates was 1/800 of the original concentrations of the additives. Fuel leaks and spills have the potential to suppress or eliminate the Streptomyces role in the soil environment or adversely alter the balance of soil micro flora. This change eventually would lead to domination of microorganisms with adverse effects on the soil environment.