The c1-t mutants of bacteriophage lambda can form prophage at 36 degrees C but cause lysis of sensitive bacteria at temperatures above 42 degrees C. Growth of cultures at 42 degrees to 46 degrees C induces prophage replication and lysis in Escherichia coli K12 (lambda c1-t); lysogenic strains containing wild-type prophage are not induced to lyse at these temperatures. Heat induction is prevented by chloramphenicol. Strains containing heatinducible prophage are much more sensitive to killing by ultraviolet light than is K12 (lambda+).