A novel TRE-binding protein complex was detected specifically in 12 out of 13 small cell lung carcinoma (SCLC) cell lines. This complex was characterised by a lower electrophoretic mobility than the 'ubiquitous' complex present in all other carcinoma cell lines analysed. As shown by UV-crosslinking and South-Western blotting, the SCLC-specific complex contains a protein with an apparent M(r) > 100 kD, which is far bigger than all Fos and Jun proteins described to date. In addition, the DNA-binding specificity of this complex is different from the specificity of the 'ubiquitous' complex or a Fos/Jun heterodimer.