This study assessed the effects of 3 commercial organic acid (OA) preparations on growth performance, intestinal morphology, cecal microbiology, and immunity of Escherichia coli K88-challenged (ETEC) broiler chickens. One thousand one-day-old male broiler chickens were divided into 8 treatments of 5 replicate pens: Negative control (NC) birds received a basal diet (BD) and were not challenged with ETEC; positive control (PC) birds fed the BD and challenged with ETEC; BD + 0.2% (S1) or 0.4% (S2) of an OA mixture (Salkil) from one to 35 d; BD + 0.1, 0.075, and 0.05% (O1) of another OA mixture (Optimax) in the starter (one to 10 d), grower (11 to 24 d), and finisher (25 to 35 d) diets, respectively, or 0.1% (O2) from one to 35 d; BD + 0.07, 0.05, and 0.05% (P1) or 0.1, 0.07, and 0.05% (P2) of a further OA mixture (pHorce) in the starter, grower, and finisher diets, respectively. All groups (not NC) were challenged with one mL of ETEC (1 × 108 cfu/mL) at 7 d of age. The 3 OA mixtures are commercial formic and propionic acid preparations. Birds challenged with ETEC (PC) had reduced (P < 0.05) growth performance, ileal morphological parameters (not crypt depth, which was increased), cecal lactobacilli, and immune responses, and increased cecal E. coli compared with unchallenged, NC birds. The addition of OA to the diets of ETEC challenged birds (S1-P2) either numerically or significantly (P < 0.05) improved growth performance, ileal morphology and immune responses, increased cecal lactobacilli, and reduced cecal E. coli. For most OA additions, the assessed parameters were generally enhanced to equivalence to NC birds. The results suggest that dietary OA supplementation can enhance the growth performance, ileal morphology, cecal microbiota, and immunity of ETEC-challenged broilers to an extent that, under such circumstances, the formulations used in this study provided similar performance and assessed parameters as non-challenged birds.