Crude cell-free extracts of Anabaena cylindrica synthesized adenosine-5'-phosphosulphate (AP(35)S) and 3'-phosphoadenosine-5'-phosphosulphate (PAP(35)S) from (35)SO4 (2-) in the presence of Mg(2+), ATP and inorganic pyrophosphatase. Maximum AP(35)S and PAP(35)S were produced at pH 7.15 and 8.05, respectively. APS kinase was detected in the supernatant of crude cell-free extracts by a spectrophotometric procedure. ATP-Sulphurylase had an absolute requirement for Mg(2+) and less than 30% AP(35)S was formed when Mg(2+) was replaced by either Mn(2+) or Co(2+). Nucleotide triphosphates other than ATP and 2'-deoxyATP were ineffective in this reaction. Maximum enzyme activity was observed at equimolar concentrations of Mg(2+) and ATP and excess of either of these was inhibitory. Other nucleotide triphosphates, like GTP, UTP, CTP, TTP, ITP, or 2'-deoxyATP also inhibited the enzyme activity. Inhibition by GTP was competitive with respect to ATP. ATP-sulphurylase activity was not affected by cysteine, methionine or glutathione.