Endogenous flows and true ileal digestibilities of amino acids were determined with the homoarginine method in growing pigs fed barley-soyabean meal or by-product-soyabean meal diets plus 50 mg/kg carbadox, or 8 g/kg formic acid (85% v/w) or without either of these additives. Seven barrows (one reserve) equipped with T-cannulae according to the steered-ileo-caecal-valve method were fed these diets in a 6 x 4 cyclic change-over design from 39 to 83 kg live weight. Both carbadox and formic acid improved the apparent ileal digestibilities of several essential amino acids in the byproduct-based diet but not in the barley-based diet. The improvements were due to reduced endogenous amino acid losses. Neither carbadox nor formic acid affected the true ileal digestibility of lysine.