Photoaffinity labelling (PAL) with [32P]8-azido-cAMP and polyacrylamide gel electrophoresis (PAGE) has been used to identify three specific cAMP-binding proteins (cAMP-BPs) within cytosols derived from the centre and periphery of 32 human colorectal cancers and from related adjacent (less than 5 cm from the tumour) and distant (more than 5 cm from the tumour) microscopically benign mucosa. By immunoprecipitation with specific anti-RI and anti-RII antibodies these proteins have subsequently been characterised as a single form of RI (48 kDa) and two forms of RII (50 and 52 kDa). The relative expression of isoforms in each specimen has been quantified by laser densitometry. There was significantly more RI expressed in both tumour centre and periphery than in either adjacent or distant mucosa (P < 0.008 by Wilcoxon signed-rank test). There was no significant difference in relative RI expression between tumour centre and periphery, or between adjacent and distant mucosa. There was no association between relative RI expression and Dukes' stage. Poorly differentiated tumours expressed significantly more RI than those that were either moderately or well differentiated (P = 0.016 by Mann-Whitney U-test). This study is the first to have characterised cAMP-BPs within human colorectal tissues and has demonstrated that colorectal cancers, and in particular those of poor histological grade, relatively overexpress RI when compared with related benign mucosa.