Heat shock in Lactococcus lactis subsp. lactis may induce as many as 16 proteins after a temperature shift from 30-degrees to 40-degrees-C. Five induced proteins were found to be immunologically related to the Escherichia coli GroEL, DnaK, DnaJ, and GrpE proteins, and to the Bacillus subtilis sigma-43 factor. From these initial studies we conclude that, in L. lactis subsp. lactis, a heat shock response similar to that known to occur in other prokaryotes might exist.