In this paper, Sr modifier (300 ppm) was added to 2L99 alloy sand castings to investigate its effect on bifilm defects in the castings. Two different sand molds were used in this study, with good and bad running system designs, to introduce different amounts of bifilm defects into the castings. The mechanical properties of the modified 2L99 castings were compared to the properties of unmodified castings and showed that with high bifilm defect contents (H) the Sr addition reduced the Weibull modulus of the UTS by 67 pct and the Position Parameter by 5 pct, and introduced a bimodal distribution into the Weibull plot of the pct Elongation. However, for castings with low bifilm defect content (L), the Weibull moduli of both the UTS and pct Elongation were significantly improved (by 78 and 73 pct, respectively) with the addition of Sr. The Position Parameter of the pct Elongation was improved by 135 pct. The results suggested that a desirable modification effect can only be achieved while the bifilm defect content in a casting was low.