The adsorption of bovine serum albumin (BSA) and gelatin onto stainless-steel surfaces was studied as a function of pH, using nonporous stainless-steel particles. The net charge equivalents of BSA and gelatin increased with increasing difference in pH values from the isoelectric points (I.E.P,’s) of the respective proteins, accompanied by charge-induced molecular expansion. BSA and gelatin adsorbed spontaneously on positively and negatively charged stainless-steel surfaces, even though the proteins had the same sign of charge as that of the stainless-steel surfaces. Adsorption isotherms similar to Langmuir-type ones with saturation plateaus were constructed for BSA and gelatin in a pH range of 3.2 to 8.0. The plateau values varied with pH, giving maxima around the I.E.P.’s of BSA and gelatin. Calculation of saturation area per BSA molecule adsorbed on stainless-steel surfaces suggests that the net charge on protein molecules is a key factor determining the packing density of protein molecules in the adsorbed layer. An increase in ionic strength from 10(-3) to 10(-1) resulted in a significant increase in the adsorbed amount of BSA but a slight decrease in that of gelatin in the isoelectric regions of the proteins. The adsorption of BSA onto stainless-steel surfaces was markedly enhanced at temperatures above 50 degrees C, beyond which BSA was progressively denatured. On the other hand, the adsorbed amount of gelatin decreased gradually from 40 to 80 degrees C. It is concluded that the adsorption behavior of proteins at stainless steel-liquid interfaces was mainly governed by the properties of solute protein molecules under changing environmental conditions, rather than by the nature of the stainless-steel surfaces.