Eighty-four Holstein calves were assigned at 2 d of age to one of three treatments: 1) control with no additives; 2) 10 g of a mixed microbial concentrate containing Lactobacillus acidophilus, Lactobacillus lactis, and Bacillus subtilis; or 3) 10 g of a B. subtilis concentrate. The microbial concentrates were mixed with milk replacer during the a.m. feeding. The milk replacer was offered twice daily at 5% BW per feeding; the reconstituted replacer contained 12.5% DM. Volume of replacer fed was based on initial weight of calf and held constant until weaning. Water and starter ration were offered for ad libitum intake throughout the trial. Calves were weaned abruptly at 30 d of age and received only water and starter from d 31 to 44. General health and performance of all calves were good. Although differences in weight gain and feed efficiency were not significant, the B. subtilis concentrate tended to have a positive effect on feed efficiency during wk 1 to 4 and on immediate postweaning gain. A higher fecal bacilli count at 6 wk in calves fed the microbial concentrates may be related to their tendency for improved gain during d 31 to 44, the immediate postweaning period.