We propose a system for detection of overproduction of protein kinase MAK-V/Hunk in tumours. MAK-V/ Hunk overproduction is observed in about 50% breast carcinomas. Positive staining is obseved in tumour cells only and has mainly cytoplasmic characteristics. Increased production of MAK-V/Hunk does not correlate with histological type of carcinoma, metastasizing, steroid receptor status (estrogen and progesterone), proliferative activity. Tumours positive for MAK-V/ Hunk were more frequently observed in c-erbB2-positive (3+) tumours than in c-erbB2-negative ones. Overproduction of MAK-V/Hunk in human breast cancer may be used fore more precise molecular typing of this tumour, in particular in case of c-erbB2-positive tumors, however, diagnostic-prognostic value of this new molecular marker needs further studies.