Protein kinase CKII (i.e. casein kinase II, CKII, NII) is expressed at a higher level in rapidly proliferating tissues and in solid human tumours (e.g. colorectal carcinomas) when compared to the corresponding non-neoplastic colorectal mucosa. This could be shown by (a) Western blotting of cellular extracts from solid tumours followed by immunostaining with an anti-CKII polyclonal antibody, (b) immunohistochemical staining of cells from tissue sections and (c) by activity measurements using the CKII-specific synthetic peptide (RRRDDDSDDD). The maximum observed activity in the colorectal carcinomas investigated was up to eightfold higher in the tumour specimens than in the non-neoplastic tissue (i.e. colorectal mucosa). The activity range was between 33-350 U/mg protein and in the case of colorectal mucosa 13-106 U/mg protein. The amount of CKII determined in the individual tumours was in the range 0.4-1.6 nmol/g tissue.