Binding of (unconjugated) bilirubin (BR) to human blood fractions has been studied using spectrofluorometry, and the results obtained were used to interpret the titration curve of BR in Whole blood as determined by front face fluorometry. It is found that BR fluorescence from whole blood reflects only that BR tightly bound to albumin the normal BR transport protein. A significant amount of BR may be bound to erythrocytes as well as to plasma proteins other than albumin, and is essentially non-fluore scent. Detergents can extract BR from all blood sites. Since BR is fluorescent in detergent micelles, total blood BR can be assayed. A significant difference between albumin-bound BR and total blood BR reflects a saturation of the albumin binding or a reduced albumin binding affinity. The reserve albumin binding capacity for BR can be obtained from the incremental fluorescence measured upon adding a saturating quantity of BR to the specimen. All three parameters (albumin-bound BR, total blood BR, and reserve albumin capacity) can be measured simply and within a few minutes on less than 100ul blood by the use of a dedicated front face fluorometer (hematofluorometer), facitating effective management of neonatal jaundice. Prototypes of the BR-hematofluorometer in two intensive care nurseries have yielded results in excellent agreement with those obtained by present, more laborious methods, which are impractical for routine use.