Abstract 1. 1. The behaviour of a series of fluorescent dyestuffs, when conjugated to protein molecules has been examined. 2. 2. Assuming the relaxation time for bovine serum albumin, the excited lifetimes of the dyestuffs vary from 0.25 to 9·10 −8 sec. Depolarisation measurements have been reported for conjugates with proteins whose molecular weights vary from 15 000 to 6500000. 3. 3. A pyrene derivative with a particularly long excited lifetime (9·10 −8 sec) shows anomalous behaviour when conjugated with bovine serum albumin under mild conditions, and the need for caution in interpreting depolarisation measurements is stressed.