Immunological cross-reactivity between Mycobacterium bovis BCG stress protein (heat shock protein: HSP64) and Meth A tumour cells was analysed by using anti-BCG HSP64 mAb whose recognition epitopes were characterized against BCG HSP64 peptides. By indirect immunofluorescence analysis (IIFA), it was found that one of seven anti-BCG HSP64 mAb, XVIIIG1, bound to the cell surface of Meth A in BALB/c mice. This result was further confirmed by western blot analysis, demonstrating the presence of a 64 kDa protein which reacted with mAb XVIIIG1 that recognizes the 110-123 amino acid peptide of the BCG HSP64. Comparison of the amino acid sequence between the mouse HSP65 and BCG HSP64 recognized by mAb XVIIIG1 revealed 50% amino acid sequence homology. It was concluded from these results that Meth A tumour cells continuously express the stress protein HSP64 as a kind of tumour-associated antigen on the surface of tumour cells.