Official Organ of the Radiation Research Society

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the copyright owner. The appearance of the code at the bottom of the first page of an article in this journal indicates the copyright owner's consent that copies of the article may be made for personal or internal use, or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the of copying, such as copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. Copy fees for pre-1985 articles are as shown on the article title pages; if no fee code appears on the title page, the copy fee is the same as for current articles. Gamma-ray-induced reversions in the Ames Salmonella tester strain TA2638 have been studied for their dependence on a number of experimental parameters. It is shown that exposure to ionizing radiations soon after plating is not the procedure that yields results which correspond to those obtained in the standard utilization of the test with chemical mutagens. The ability to detect mutants is improved by irradiation 6 hr after the beginning of the incubation of the plated bacteria. This procedure has the double advantage of a markedly increased ratio of radiation-induced to spontaneous revertants and of resulting in substantial insensitivity to fluctuations in the number of bacteria initially plated. The reversion-doubling dose so obtained is 1.3 Gy; i.e., it is sufficiently small to disregard inactivation of the bacteria.