Zinc, copper and iron utilization was examined in rats fed diets containing 30% lactalbumin (L); 30% soy assay protein (S) or 30% soy assay protein supplemented with 0.26% cysteine (SC), 0.45% methionine (SM), 0.26% cysteine and 0.45% methionine (SCM) or 0.71% cysteine (SXC). Diets L, SC and SCM contained equal amounts of cysteine; diets L, SM and SCM contained equal amounts of methionine; diets L, SCM, SXC contained equal amounts of sulfur-containing amino acids. Rats fed diet L had significantly higher levels of zinc in tibias, kidneys and plasma; higher levels of copper in kidneys; and higher levels of iron in tibias than rats fed diet S. Rats fed diet L also absorbed (apparent and true) significantly more zinc; excreted significantly more zinc of endogenous origin in the feces; and absorbed (apparent) significantly less copper than rats fed diet S. Rats fed the soy diets supplemented with sulfur-containing amino acids, especially diet SXC, tended to have elevated levels of zinc in their tibias and kidneys and greater apparent and true absorptions of zinc than rats fed diet S. Rats fed diet SXC still had significantly lower zinc levels in tissue than rats fed diet L. Apparent absorption of zinc was similar among rats fed diets L, SCM and SXC.