The influence of pH, temperature, ionic strength, and protein modification on the sorption (moles of chemical bound per mole of protein) of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) and 3′,4′-dichloropropionanilide (propanil) to bovine serum albumin (hereinafter referred to as BSA) was examined. Free amino groups of BSA were involved in the binding of both diuron and propanil. In addition, tryptophanyl residues appeared to be involved in the binding of propanil. Studies made with derivatives of diuron suggested that the amide hydrogen and carbonyl oxygen of the phenylamide are involved in the binding mechanism. Conformation of the protein was suggested to control the extent of binding. Increased chlorination of the phenyl ring was correlated with increased binding onto BSA. Propanil was bound to a greater extent than diuron by the protein.