Cyclic adenosine 3′,5′-monophosphate (cAMP) activity was observed inStreptococcus lactisC2,Streptococcus cremorisC10,Streptococcus diacetlactis18-16, andStreptococcus thermophilusC3.In vitroassays of cell-free extracts obtained fromS.lactisC2 showed that the cAMP-associated enzymes adenyl cyclase and phosphodiesterase were also present.In vitroexperiments showed that prostaglandin E1(PGE) stimulation of adenyl cyclase increased cAMP concentrations approximately fivefold, andin vivostudies showed that PGE treatment ofS.lactisC2 increased intracellular cAMP concentrations twofold. Furthermore, PGE-induced elevation of intracellular cAMP levels was shown to prevent the repression of β-D-phosphogalactoside galactohydrolase synthesis by glucose.