Although luminous bacteria were among the very first of bioluminescent organisms to be made the subject of scientific research (Boyle 1667a,b), only recently has a satisfactory answer to one of the most puzzling aspects of bacterial luminescence become firmly established (McCapra and Hysert 1973, Dunn et al. 1973, Shimomura et al. 1972, 1974, Vigny and Michelson 1974). This aspect pertains to the chemical identity and function of what might be called "bacterial luciferin," comparable or equivalent to the specific substrate in well-defined substrate-