Field observations suggest that propionic acid-based mold inhibitors reduce the severity of proventriculitis and consequent fragile intestines. Therefore, two studies were conducted to determine the effects of a propionic acid-based mold inhibitor (Mycocurb), calcium propionate, and propionic acid on intestinal strength. Cobb x Cobb male broiler chicks were placed in an experimental design that consisted of six dietary treatments with four replicate floor pens of 40 (Experiment 1) and 30 (Experiment 2) broilers per pen. The dietary treatments in Experiment 1 consisted of control, 2.27, 4.54, and 9.07 kg/ton Mycocurb and 4.54 and 9.07 kg/ton calcium propionate. The dietary treatments in Experiment 2 consisted of control, 2.27, 4.54, and 9.07 kg/ton Mycocurb, and 4.54 and 9.07 kg/ton propionic acid. Water and the dietary treatments were made available to the broilers for ad libitum consumption from 1 d to 6 wk of age. In Experiment 1, five broilers from each pen were killed each week, a 10-cm section of intestine was removed anterior to the cecal junction, and its strength was measured. The contents of this section of intestine were collected and pH determinations were made on this material. In Experiment 2, five broilers were killed from each pen starting when the broilers were 3 wk of age, and every week thereafter a 10-cm section of intestine was removed posterior to the duodenal loop and its strength measured. The contents of this section of intestine were collected and pH determinations were performed on this material.(ABSTRACT TRUNCATED AT 250 WORDS)