The study of genes and their functions are very important in order to gain insight into the molecular mechanisms that lead to diseases in humans. The use of small interfering RNAs, siRNAs, has great importance in studying the gene function and it is very successfully applied in genetic manipulation procedures. They are successfully used to prevent gene expression in the level of mRNA blocking its translation into protein. In this study, it was tested the efficiency of suppressing expression of ribosomal protein S6 (RPS6) with specific siRNA molecules. The efficiency of silencing was determined 48 hours after transfection with siRNA against RPS6 in A549 cell lines and three concentrations were tested to determine the one that best prevents expression of ribosomal protein S6. The efficiency of siRNA against RPS6 was determined by using the method polymerase chain reaction in real-time to measure the expression of mRNA for RPS6 and using the Western blot method to analyse the expression of RPS6 protein. The results show that siRNA against RPS6, at concentrations of 40 nM and 60 nM, very successfully leads to reduced expression of mRNA for RPS6 and consequently reduced expression of RPS6 protein. Therefore, the tested siRNA is highly effective in preventing expression of RPS6 and may be used for experimental purposes for the study of ribosomal protein S6 function.