Mutant TLT2, obtained by insertion of a lux-bearing transposon in Anabaena sp. strain PCC7120, increases in luminescent intensity when transferred from 30-degrees to 20-degrees-C. The addition of different types of antibiotics to the culture medium showed that some of those known to have the ribosome as target led to an increase of luminescence of TLT2 at 30-degrees-C, in accord with the idea that ribosomes may, be sensors of cold shock. Furthermore, rifampicin, which destabilized luminescence at 20-degrees-C, decreased the rate of loss of luminescence upon transfer of TLT2 from 20-degrees to 30-degrees-C; this suggests that luminescence of TLT2 is transcriptionally controlled by a repressor at 30-degrees-C.