Hexokinase isoenzyme composition has been noted to vary in different tissues and with the developmental and metabolic status of the cell. Until now these investigations were performed either by isoenzyme electrophoresis or by column chromatography. In this report we described an RNA-PCR method to evaluate the percentage of HK-1 and HK-2 in different rat tissues. Furthermore we applied the method to determine if a shift in isoform composition is detectable in human renal carcinomas compared to normal kidney tissue. In all of our specimens we were able to detect a shift toward HK-2 in the carcinoma specimens. We discuss a possible role for the detection of the shift in isoenzyme composition as a possible marker to discriminate between normal and malignant specimens.