Native human serum albumin (HSA) is not chemotactic for neutrophil leucocytes. Certain treatments such as acidification or reduction-alkylation may cause HSA preparations to become moderately chemotactic, whereas, after other treatments such as heat-denaturation, very little increase in chemotactic activity is observed. The chemotactic activity which follows denaturation is generally confined to monomeric forms of HSA. Acquisition of such activity correlates well with physical measurements of conformational change in the HSA molecule inasmuch as the viscosity and surface activity of chemotactically active HSA preparations are greater than those of preparations without activity. A hypothesis is proposed whereby chemotactically active sites on protein molecules are associated with hydrophobic areas of the molecule and are therefore concealed in the native state, but may become exposed during unfolding so that denatured proteins acquire chemotactic activity.