The effect of dietary citric acid (CA) and fumaric acid (FA) on pig weight gain (ADG) and gain/feed (G/F) was studied in two trials using 192 crossbred, 4-wk-old weaning pigs. Three dietary levels (0, 1.5 or 3.0%) of either FA (Trial 1) or CA (Trial 2) with or without an antibiotic supplement (110 mg chlortetracycline, 110 mg sulfamethazine and 55 mg penicillin/kg diet) formed six treatment combinations in each trial. These six diets were fed to two replicate pens of eight pigs each for a 4-wk period. In Trial 1, ADG was improved (P less than .01) during wk 1, and G/F was improved during wk 1 (P less than .01) as well as during wk 1 to 2 (P less than .05) for pigs consuming FA-supplemented diets. In Trial 2, CA had no beneficial influence on ADG during the 4-wk trial. However, feed intake during wk 1 was depressed (P less than .05) by adding CA, as was G/F during wk 1 to 2 (P less than .05). Based on these results, FA was selected to be used in a nutrient balance study. Twelve 4-wk-old weanling pigs were fed one of three diets: control (C), C + 1.5% FA, or C + antibiotic supplement (A). Diet digestible energy (DE), ME and N-corrected ME (MEN) were not different among treatments. Nitrogen balance, percentage N retained and apparent N digestibility were not affected by dietary treatment. Calcium balance and percentage of Ca retained were unaffected by diet.(ABSTRACT TRUNCATED AT 250 WORDS)