Sindbis virus strain AR339 induces interferon by 3 h after infection at 39 degrees C and by 8 h after infection at 30 degrees C. The time course of interferon induction between 4 and 9 h after infection at the restrictive temperature (39 degrees C) was measured for 24 temperature-sensitive (ts) mutants, all of which induced interferon by 16 h after infection. Mutants showing total RNA synthesis at 39 degrees C greater than 10% of the wild-type level induced interferon with kinetics similar to the wild-type. Of those mutants showing 1 to 10% of the wild-type level of RNA synthesis at 39 degrees C, four induced interferon with kinetics similar to the wild type, whereas six showed a lag in induction. Nine mutants, showing 0 to 5% of the wild-type level of RNA synthesis at 39 degrees C, and the wild type, were examined for single and double-stranded RNA synthesis at 30 and 39 degrees C, using a combination of lithium chloride precipitation and CF11 cellulose chromatography. Six of these mutants showed a lag in interferon induction at 30 degrees C, while three showed no lag. For all nine mutants, double-stranded RNA synthesis at 39 degrees C was undetectable, although easily detectable for the wild type. Both the wild type and the mutants synthesized double-stranded RNA at 30 degrees C. For all mutants, time of interferon induction at 39 degrees C was correlated with c.p.e. as measured by trypan blue uptake 30 h after infection. The mutant F104, showing undetectable RNA synthesis and a long lag in interferon induction at 39 degrees C, was examined for interferon induction in a temperature shift system. Only 1/2 h at 30 degrees C, before a shift to 39 degrees C, was required for interferon to be induced with wil-type kinetics, and this was correlated with an increased c.p.e. 30 h after infection. Increased RNA synthesis and infectious virus production were detectable at 30 degrees C after an initial hour at 30 degrees C. It is concluded that, for interferon to be induced with normal kinetics, early virus functions are required, but that normal levels of double-stranded RNA synthesis are not necessary. The events which lead to interferon induction also lead to visible c.p.e.