Sixty high-producing multiparous Holstein cows were used in a randomized complete block design experiment to determine the effect of a direct-fed microbial (DFM), Megasphaera elsdenii NCIMB 41125 (Me), on dairy cattle productivity and health. The cows received one of two experimental diets (total mixed rations (TMR)) from calving until 60 days post partum. Two experimental diets were formulated, namely a low concentrate diet (40% roughage : 60% concentrate) and a high concentrate diet (30% roughage : 70% concentrate) that were fed as a TMR for the duration of the trial. The low and high concentrate diets contained respectively 181 g/kg and 178 g/kg of crude protein, 448 g/kg and 504 g/kg of non-fibre carbohydrate, 282 g/kg and 238 g/kg of neutral detergent fibre and 42 g/kg and 43 g/kg of ether extract. There were four experimental treatments, namely: (1) Low concentrate diet control (LCC), (2) Low concentrate diet and dosed with Me (LCD), (3) High concentrate diet control (HCC) and (4) High concentrate diet and dosed with Me (HCD). Dosing with Me did not show any advantage, regardless of level of concentrate. Dry matter intake, milk production, milk composition, feed efficiency, body mass and body condition score were not affected by treatment. Furthermore, treatment did not affect rumen pH, rumen lactic acid or volatile fatty acid concentrations. Faecal pH, however, was statistically significantly higher, and faecal starch content significantly lower in cows dosed with Me. Lack of results suggests that further research is needed on dose time and/or frequency as well as the option to use it as an infeed product.