Adsorption onto chromium surfaces during heat treatment (65-68 degrees C) of beta-lactoglobulin A and B in phosphate buffer, pH 6.88, was investigated by in situ ellipsometry. Thermal unfolding and in situ heat-induced aggregation under the same conditions were studied by differential scanning calorimetry and dynamic light scattering, respectively. A lag phase was observed for buildup of the thickness of the adsorbed layer of beta-lactoglobulin A and B at 68 degrees C. The lag period was found to increase with decreasing temperature, The difference in adsorption rate between the variants was found to be in agreement with the aggregation difference in solution, that is, the A variant aggregated more rapidly after a somewhat slower beginning. These results could be related to a difference in the thermal unfolding behavior, By applying a non-two state model the second of two transitions was suggested to involve the activation of the free sulfydryl group. The results indicated that fouling proceeds via attachment of aggregates with an activated free sulfydryl group through an interchange reaction with a disulfide bond in the adsorbed protein layer.