Unlike its conjugated (direct acting) counterpart, unconjugated (indirect acting) bilirubin is insoluble in water, but highly soluble in lipids. Hence, it tends to diffuse out of plasma into the lipid-rich CNS with resultant kernicterus. This does not occur when unconjugated bilirubin is bound to albumin, as the complex is too large for diffusion. The production of kernicterus thus involves the increase of free, unbound, unconjugated bilirubin in plasma, either by dissociation from its albumin binding sites or by the introduction of one or more anions which compete preferentially for a common or shared site, thus displacing the bilirubin from its albumin bond. FACTORS AFFECTING BINDING Lowering the pH promotes bilirubin-albumin dissociation,1 and acidosis has thus been implicated as a factor in kernicterus occurring at low levels of serum bilirubin.2-5 In addition, competition for bilirubin binding sites is exhibited by a number of endogenously occurring substances as well as exogenously administered agents. Both hematin (increased in hemolytic states) and the nonesterified fatty acids (increased under conditions of both hypothermia6 and hypoglycemia7 are capable of displacing bilirubin from its albumin binding sites. In addition, a number of drugs, among which sulfisoxazole (Gantrisin) is the most widely known in view of both its clinical8 and experimental9 production of kernicterus in this fashion, are capable of causing similar displacement in vitro. Among the techniques employed to demonstrate an increase in free bilirubin is the measurement of displacement of spectral curves.10,7 The technique depends on a difference in absorbance of free (420 to 440 nm) versus bound (460 to 465 nm) bilirubin, with changes in the shapes of the curves as the amounts of free and bound bilirubin are altered.