The functional half life of alpha-amylase mRNA was investigated in Bacillus amyloliquefaciens CCM 3502. The amount of mRNA was followed indirectly: the amount of protein translated by preformed mRNA was-measured after transcription was terminated by rifampicin. The radioactivity of C-14-leucine-labeled extracellular protein produced by drug-treated cells was a measure of the quantity of alpha-amylase, because this amount formed, in standard experiments, at least 90% of the total extracellular proteins. The effect of rifampicin and chloramphenicol was studied at the level of mRNA transcription and translation. Negligible radio-activity of the extracellular protein produced by cells after the addition of chloramphenicol (200-mu-g.ml-1) proved that alpha-amylase is formed de novo, and no form of preformed enzyme is secreted. From the radioactivity of extracellular proteins synthesized after the inhibition of mRNA by rifampicin, the half life of alpha-amylase mRNA was calculated to be 26.9 min at 30-degrees-C. The half life of mRNA for cellular proteins was calculated to bc 2.9 min.