In view of the facts that the normal intestinal flora exerts beneficial effects and that bifidobacteria are a more important component in the breast-fed than in the bottle-fed infant, factors affecting the growth of the latter microorganisms are of interest. A series of transition and other metals were shown to be growth inhibitors ofBifidobacterium bifidumvar.pennsylvanicus.Such inhibition could be reversed fully or partially by 0.5–1.0 mMFe2+in the case of Zn2+, Cu2+, Au3+, Pt4+, La3+, Cr3+Mn2+, Ni2+, and Cd2+, but not with Ag+, Hg2+, and VO2+. In addition, 2–4 mMCa2+substantially relieved the inhibitory effects of Zn2+, Mn2+, and La3+, and partially relieved the effects of Cd2+. Mg2+was ineffective in relieving Zn2+inhibition, but Ba2+and Sr2+could replace Ca2+to some extent. The calcium metabolism antagonists verapamil, ruthenium red, 2-chloroadenosine, lasalocid, Ca-ionophore A-23187, and calmodulin inhibitors W-5 and W-7 inhibited microbial growth. Inhibition could be relieved fully or partially with 0.5–1 mMFe2+. Mg2+relieved the inhibition by lasalocid, Ca-ionophore A-23187, and verapamil, whereas Ca2+was effective only in the case of Ca-ionophore A-23187. We conclude that calcium and magnesium fluxes play an important role in the physiology of the bifidobacteria and that several metal growth inhibitors interfere with iron metabolism.