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Poliovirus ts mutants were cultivated at a temperature (39·6°) too high for their optimal growth but low enough to permit full growth of the parental ts + strain. Tests for production of infectious RNA and serum-blocking antigen showed a gradient of mutant productivity that corresponded closely with an earlier physiological classification of these mutants. No mutant produced fully wild-type yields of either RNA or antigen. In general, group A mutants produced the least RNA and antigen (some yields being undetectable or < 1 % of ts + values) while group D mutants produced the most (up to 88 % of ts + RNA and 27·6 % of ts + antigen). Mutants of groups B and C produced intermediate yields. Six of the mutants (all that were classified belonging to groups A or B) converted their small yield of RNA and antigen to infective virus with an efficiency not significantly different from that of wild-type virus. The remainder (thirteen mutants, most of those classified belonging to groups C or D) matured either or both their RNA and antigen inefficiently and accordingly exhibited a defect in some stage of maturation or assembly.