Abstract— Ultraviolet irradiation of DNA in vitro and in vivo results in the formation of many different types of photoproducts. The general procedure for correlating such photoproducts with photo‐biological effects is outlined and is applied for cyclobutane pyrimidine dimers and for spore photoproducts. La irradiación de DNA in vitro e in vivo conduce a la formación de distintos tipos de fotoproductos. Es delineado el procedimlento general para relacionar esos fotoproductos con efectos biológicos y es aplicado a dimeros de ciclobutano, pirimidina y para fotoproductos de esporas. Copyright © 1968, Wiley Blackwell. All rights reserved