Bacteriophage T3-induced RNA polymerase is rapidly inactivated at 42 degrees C. Addition of T3 DNA delays this process for 30 s and reduces the rate with which the enzyme activity is lost indicating that a labile binary complex between T3 DNA and polymerase must have been formed. The ternary complex between T3-specific RNA polymerase, T3 DNA, and nascent RNA chains obtained when the enzyme is incubated with T3 DNA, GTP, ATP, and UTP is stable to heat (42 degrees C) and only slowly inactivated by polyvinyl sulfate. The optimal temperature for the formation of polyanionresistant ternary complexes is 30 degrees C while the elongation of T3 RNA chains proceeds fastest at 38 degrees C.