We report on magnetic-field effects in illuminated tetracene field-effect transistors. A decrease in the photocurrent is observed when an external magnetic field is applied. In this case, the resistance of the conducting channel increases by up to 0.75%, which leads to positive magnetoresistance. This can be attributed to: (1) A magnetic field-induced decrease in the interaction rate between triplet excitons and trapped holes. (2) The coupling between the singlet and the triplet manifold of states is decreased in a magnetic field and, hence, the singlet exciton fission and triplet exciton fusion rate constants are reduced.