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Under commercial conditions and during production periods, pigs are generally exposed to a number of stressors that may have direct or indirect influence on their performance and general health. As a result, environmental stressors can influence the productivity as well as the onset and severity of infectious diseases. The aim of this study was to investigate the effects of crowding stress on performance and some immunological responses in piglets challenged with enterotoxigenic Escherichia coli (ETEC) K88+ and fed diets containing anti-ETEC probiotic strains derived from E. coli in a 3-wk trial. Ninety-six piglets at 20 ± 1 d of age were randomly assigned to 6 experimental treatments with 3 pigs/pen and 5 replicates/treatment and fed a basal mash diet. To model stress, piglets were housed in 2 kinds of pens: standard floor allowance (SFA; no stress) and half standard floor allowance (HSFA; stressed). The treatments were 1) control pigs with SFA, 2) control pigs with HSFA, 3) pigs with SFA and challenged with ETEC (ESFA), 4) pigs with HSFA and challenged with ETEC (EHSFA), 5) pigs with ESFA and supplemented with a cocktail of E. coli probiotics (PSFA), and 6) pigs with EHSFA and supplemented with a cocktail of E. coli probiotics (PHSFA). After 4 d acclimatization, crowding stress was started on d 5, E. coli probiotics were given daily from d 5, and ETEC challenge was administered on d 8 and 10. Body weight and feed disappearance were determined weekly and severity of diarrhea was characterized daily. Blood samples were collected for biochemistry and inflammatory analysis and pigs were euthanized to obtain digesta for bacterial enumeration. The ADG and ADFI decreased (P < 0.05) in stressed and ETEC-challenged pigs 5 d after ETEC challenge. The ETEC population was higher in both ileal and colon digesta of stressed pigs whereas ETEC-challenged and probiotic-fed pigs had higher fecal consistency scores 96 h and 7 d after ETEC challenge. Stressed pigs had higher (P < 0.001) levels of neutrophils, neutrophihlymphocyte ratio, albumimglobulin ratio, fibrinogen, total proteimfibrinogen ratio, and Cortisol. Generally, the levels of Cortisol, interleukin 6 (IL-6), and tumor necrosis factor alpha (TNF-α) differed over time among some treatment groups. In conclusion, reduced space allowance stress and ETEC challenge depressed certain aspects of performance and immunological responses and increased E. coli counts; however, probiotics feeding had little positive effects on these measures suggesting further research. [ABSTRACT FROM AUTHOR]