Purpose: to evaluate the effect of Streptococcus mutans on the microleakage around the margins of cast metal crowns cemented on the prepared abutments with different types of currently used dental luting cements.Materials and methods: 45 intact human maxillary premolars were prepared to receive complete metal crowns. Teeth with their crowns were then randomly divided into three groups according to the type of cement used for cementation of the crowns; Group (A) crowns cemented with Zinc phosphate cement(Adhesor), Group (B) crowns cemented with Glass ionomer cement(Kavitan cem), and Group (C) crowns cemented with Self adhesive resin cement(RelyX unicem). Marginal gaps of the cemented crowns were checked under x63 magnification using stereomicroscope. Each group then subdivided into three subgroups according to the media that it was immersed in, media are: Brain heart infusion broth (BHIb) without bacteria, (BHIb) with low bacterial concentration, and (BHIb) with high bacterial concentration. All subgroups were then placed in an incubator at 37oC for 7 days and then removed, and washed. The PH of the media was measured using PH meter. Samples then immersed in 2% methylene blue for 24 hours. Teeth then rinsed and sectioned mesiodistally and assessed under magnification for microleakage. A five-point scale was used to score degree of microleakage. Results revealed that streptococcus mutans had significant effect on microleakage. There is positive relation between marginal gaps and microleakage. Adhesor cement had the highest microleakage scores while Rely X unicem had the lowest one.