The work reported in this paper forms part of a programme of investigation into the normal parameters of radiosensitive tissues in the plaice. The work was conducted prior to investigations into the effectsof ionizing radiations on these tissues, in order to form a backgroundto examinations of fish from areas of radioactive pollution resulting from the controlled disposal of radioactive effluent. Tissues were selected on the basis of known radiosensitive tissues in mammals and the work on red blood values reported here arose from these preliminary investigations.