Gamithromycin is an antimicrobial agent which can inhibit the growth of MmmSC strains B237 and Tan8 with MICs of 0.00012 and 0.00006 μg/mL, respectively. IC50 & Target: MIC: 0.00012 μg/mL (MmmSC strain B237), 0.00006μg/mL (MmmSC strain Tan8)^[1] In Vitro: The MIC values in serum are significantly lower than those in artificial medium; at an initial inoculum size of 10⁶ cfu/mL, these are 64-, 8- and 64-fold lower for gamithromycin, tylosin and tilmicosin, respectively, against MmmSC strain B237 in serum compare to artificial medium. A similar pattern emerges for Tan8. Heat-inactivation of serum results in an MIC for gamithromycin that is higher than in either non-treated serum or artificial medium^[1]. In Vivo: The proportion of foals that recover without the need for a change in treatment is significantly (P<0.048) higher for foals treated with Gamithromycin (GAM) (38 of 40; 95%) or AZM-RIF (39 of 40; 98%) compare to control foals (32 of 41; 78%). The clinical scores, number of abscesses and the abscess scores after 1 and 2 weeks of treatment are significantly lower for foals treated with Gamithromycin (GAM) or AZM-RIF compare to control foals. The WBC count of foals treated with Gamithromycin (GAM) is significantly higher than that of foals treated with AZM-RIF on week 3 of treatment^[2].