Abstract 1. 8-Azido-ATP is a substrate for Escherichia coli (Ca 2+ + Mg 2+)-ATPase ( E. coli F 1). 2. Illumination of E. coli F 1 in the presence of 8-azido-ATP causes inhibition of ATPase activity. The presence of ATP during illumination prevents inhibition. 3. 8-Azido-ATP and 4-chloro-7-nitrobenzofurazan (NbfCl) bind predominantly to the α subunit of the enzyme, but also significantly to the β subunit. 4. The α subunit of E. coli F 1 seems to have some properties that in other F 1-ATPases are associated with the β subunit.