We evaluated feather meal (FTH) and poultry by-product meal (PBM) as complementary protein sources for growing calves. In a replicated 84-d growth trial, individually fed steer calves (n = 120; 252 +/- 24 kg) were supplemented with urea or with graded levels of soybean meal (SBM), FTH, PBM, or 2/3 FTH:1/3 PBM (CP basis). Protein efficiency, calculated as gain above the urea control vs natural protein intake using the slope-ratio technique, was greater for FTH than for SBM, PBM, and 2/3 FTH:1/3 PBM (P < .10). Addition of ruminally protected methionine and lysine did not affect protein efficiency (P > .30) for FTH, PBM, or 2/3 FTH:1/3 PBM. Even though true protein digestibility in the gastrointestinal tract in a trial with lambs was similar (P > .15) for FTH (83.1%) and PBM (91.2%), escape protein was greater for FTH (66.8%) than for PBM (43.6%). Analyses were conducted to estimate intestinal flow of amino acids relative to requirements for live animal gain, and no obvious amino acid deficiencies were present. The lack of a response in protein efficiency to ruminally protected methionine and lysine suggests that FTH and PBM are adequate in these amino acids. Although FTH and PBM are excellent sources of metabolizable protein, there was no complementary response in protein efficiency between them.