Microsequencing of a polypeptide with MW of 14.5 and pI of 5.0 induced by heat treatment at 42 degrees C and 50 degrees C in Lactococcus lactis subsp. lactis revealed that it corresponds to the co-chaperonin GroES. Quantitative analysis of analytical 2-D gels showed a relative induction of 12- and 11-fold after 30 min of heat adaptation at 42 degrees C and 50 degrees C, respectively. GroES is also induced by an acid shift from pH 7 to pH 5.5 and by UV254 nm-irradiation, with relative induction factors of 3.8 and 2.3, respectively. To our knowledge this is the first report showing induction of GroES by mild acid treatment. Contrasting to the relative induction of the groEL gene product, the second protein encoded by the groESL operon, GroES shows significantly higher induction under all stress situations.