A 2 x 2 factorial design with treatments of 0 and 2.5 microgram/g aflatoxin and 0 and 2.0 microgram/g ochratoxin A was used to evaluate the individual and combined effects of these mycotoxins on bruising and blood thigh syndrome in broiler chickens. Trial 1 consisted of four replicate pens of 10 broilers per replicate, which were maintained on these dietary treatments from 0 to 3 weeks of age. At 3 weeks of age two replicate pens per treatment were randomly selected and placed on toxin-free feed with two replicate pens remaining on toxin feed until they were 6 weeks of age when the trial was concluded. In Trial 2 three replicate pens of 38 broilers per treatment were maintained on toxin feed from 0 to 3 weeks of age. At 3 weeks of age 13 broilers per replicate were placed on toxin-free feed with 25 broilers per replicate remaining on toxin until they reached 7 weeks of age when Trial 2 was concluded. These data indicate that a synergistic toxicity exists between aflatoxin and ochratoxin A to significantly (P less than .05) decrease body weight. Body weights of broilers on aflatoxin or ochratoxin A diets for only 3 weeks partially recovered by 6 or 7 weeks of age; however, the body weights of broilers on the interaction treatments for only 3 weeks remained significantly (P less than .05) depressed at 6 and 7 weeks of age.(ABSTRACT TRUNCATED AT 250 WORDS)