WE have reported before1 a method for the isolation of conditional lethal mutants (temperature sensitive) of mammalian cells and have shown2,3 the usefulness of such mutants in the study of cell biology. We modified the technique originally designed to select for mutant clones restricted in growth at 39.5° C to select preferentially for mutants defective in (1) DNA synthesis (involving replication), or (2) RNA synthesis (involving transcription or synthesis of various RNA species). These mutants were intended for studying (1) cellular regulatory mechanisms and (2) host-virus interactions. Because of unusually high reversion frequency in the mutant clones of the tetraploid cell line (in the order of 10−4 to 10−5), a second selection (by testing these mutants through successive passage for efficiency of plating at the restricted temperature) becomes necessary so that these selected clones could be used for biochemical analysis. All our mutants were thus selected for reversion frequencies of less than 10−5.