The groES gene of Mycobacterium avium strain 485 was cloned and expressed in Escherichia coli and the recombinant GroES protein was purified by affinity chromatography. The GroES preparation showed high purity by electrophoresis and immunoblotting. Immuno-electron microscopy showed that GroES was located both in the cytoplasm and on the surface of the mycobacterial cells and thus is readily available to interact with the host immune system. BALB/c mice were immunised intranasally with recombinant GroES, alone or in combination with a synthetic oligodeoxynucleotide containing unmethylated CpG motifs, and tested for protection against infection with M. avium. Neither GroES nor CpG alone provided any protection against subsequent challenge with M. avium, whereas a combination of the two significantly protected the lungs and spleen against colonisation by M. avium after intranasal challenge with a low dose of the organism. This indicates that intranasal administration of GroES and CpG oligodeoxynucleotides increases the resistance of BALB/c mice to M. avium infection.