In Exp. 1, 240 beef steers (initial BW = 332.8 kg) were used to determine the effects of Lactobacillus acidophilus (LA) plus Propionibacterium freudenreichii (PF) on performance, carcass, and intestinal characteristics; serum IgA concentrations; and the prevalence of Escherichia coli O157 (EC). Cattle were fed a steam-flaked corn-based, 92% concentrate diet, and the four direct-fed microbial (DFM) treatments (12 pens/treatment) included in a randomized complete block design were as follows: 1) control, lactose carrier only (CON); 2) 1 x 10(9) cfu of LA NP51 plus 1 x 10(6) cfu of LA NP45 plus 1 x 10(9) cfu of PF NP24 per animal daily (LA45-51H); 3) 1 x 10(9) cfu of LA NP51 plus 1 x 10(9) cfu of PF NP24 per animal daily (LA51); and 4) 1 x 10(6) cfu of LA NP51 plus 1 x 10(6) cfu of LA NP45 plus 1 x 10(9) cfu of PF NP24 per animal daily (LA45-51L). No differences (P > 0.10) were detected for pen-based performance data. The average lamina propria thickness for LA51 and LA45-51H steers was less (P = 0.02) than the average for CON and LA45-51L steers. Moreover, LA51 and LA45-51H steers had a lower (P = 0.06) prevalence of EC shedding than CON and LA45-51L steers. In Exp. 2, 660 steers fed 91% concentrate, steam-flaked corn-based diets were used to determine the effects of the following DFM treatments (10 pens/treatment) on performance, carcass, and intestinal characteristics: 1) control, lactose carrier only (CON); 2) 5 x 10(6) cfu of LA NP51 plus 5 x 10(6) cfu of LA NP45 plus 1 x 10(9) cfu of PF NP24 per animal daily (LA45-51L); and 3) 1 x 10(9) cfu of LA NP51 plus 5 x 10(6) cfu of LA NP45 plus 1 x 10(9) cfu of PF NP24 per animal daily (LA45-51H). Steers were from two weight groups (WG). One group (SDOF; BW at arrival = 351.5 kg) had grazed before arrival, and the other group (LDOF; BW at arrival = 314 kg) had been in a grower yard. A split plot was used with WG as the whole-plot factor and DFM in the split plot. There was an interaction of WG and DFM for ADG (P = 0.05) and for carcass-adjusted ADG (P = 0.08). The simple-effect ADG and carcass-adjusted ADG means for DFM treatments differed (P < or = 0.01) between WG classifications. Within SDOF, ADG for CON and LA45-51L did not differ (P = 0.70), but both were less (P < or = 0.08) than for LA45-51H. Overall, these data indicate that live cultures of LA plus PF did not greatly affect feedlot performance and carcass characteristics. Some of the DFM used decreased fecal EC shedding, which might be related to the results for ileal lamina propria thickness.