Phone/Fax: þ36 62 544121, Web: www2.sci.u-szeged.hu/ant/ Since their discovery, carbon nanotubes (CNTs) have attracted intense attention to broad range of potential applications. In contrast to the 1D isolated single-walled carbon nanotubes (SWCNT), 2D films or bundles made of thousands of tubes have been introduced as more advantageous building blocks for new types of applications in mechanically flexible and stretchable, optically transparent electronic systems. In our experiments, we combined photosynthetic reaction centre proteins, the light energy converter units in living cells, purified from purple bacteria, with multiwalled carbon nanotube (MWCNT) bundles. The change in the conductivity of the bare MWCNT bundles and the RC/MWCNT composite after light excitation was measured and compared. We found that the electrical conductivity under light excitation depends on the intrinsic conductivity of individual tubes within the bundles and on structural characteristics, like geometry (diameter, length, spatial arrangement, interconnects, etc.) and the electronic coupling with the RCs.