Utilizing a suitable irradiating transmission line chamber, the effects of 900 MHz radio frequency fields on DNA mutability and repair in Escherichia coli strains were investigated. The transmission line was very versatile and able to easily apply different field values. In this experiment the maximum electric and magnetic fields were 66 V/m and 260 nT, respectively, in the absence of cell plates. Slight decrease in spontaneous mutability to erythromycin resistance was demonstrated in mismatch-repair proficient bacteria exposed to the radio frequency electromagnetic field (RF-EMF) during their growth on solid medium. The anti-mutagenic effect of the RF-EMF was much more impressive at the level of a hypermutagenic cytosine repeat, whose stability is strongly dependent on the activity of the mismatch repair system. In contrast, in mismatch repair-defective background RF-EMF neither affected the general DNA mutability nor the stability of the cytosine repeat, suggesting that the anti-mutagenic effect of the 900 MHz RF-EMF might be due to an improved efficiency of the mismatch repair system.