Thin films of metalloprotein, deposited directly onto carbon-formvar-coated electron microscope grids, provide a potentially useful, but little developed, type of standard for biological transmission X-ray microanalysis. The possibility of using various metalloproteins was investigated in terms of their suitability as standards, and compared with an evaporated film of gelatin-salt mixture. The metalloproteins that were selected comprised conalbumin (containing detectable S, Fe), hemocyanin (S, K, Ca, Fe and Cu), insulin (S, Zn), phosvitin (P, Ca, Fe, and Zn) and α-lactalbumin (S, Ca). In each case, the metalloprotein preparation was homogenous at the microlevel (unlike evaporated gelatin-salt mixtures), and was stable in the electron beam upto a current of 120 nA, over a lifetime of 200 s.