THEFT OF LIBRARY MATERIALS IS A PROBLEM PLAGUING PUBLIC, PRIVATE, AND INSTITUTIONAL LIBRARIES THROUGHOUT THE UNITED STATES AND RESULTS IN EXPENSIVE REPLACEMENT COSTS. LIBRARY THIEVES INCLUDE DOCTORS, LAWYERS, COLLEGE STUDENTS, AND PROFESSIONAL THIEVES WHO SPECIALIZE IN RESELLING STOLEN LIBRARY BOOKS AND MATERIALS. THEFTS FROM LIBRARIES MAY INVOLVE BOOKS, FILMS, RECORDS, PAINTINGS, EVEN LIGHTBULBS AND ASHTRAYS. A MAJORITY OF PUBLIC LIBRARY DIRECTORS RATE THEFT AS THEIR FOREMOST SECURITY PROBLEM. LOSS ESTIMATES FROM 140 LIBRARIES AVERAGED OVER $3,000 EACH PER YEAR. SINCE MANY LIBRARIES DO NOT HAVE AN INVENTORY SYSTEM, THE TRUE EXTENT OF THE COST OF THEFT IS DIFFICULT TO DETERMINE. A SERIES OF 5-PERCENT SAMPLES OF RANDOM SHELF SECTIONS CAN PROVIDE AND ADEQUATE BASE FOR ESTIMATING THEFT LOSSES. USE OF SUCH AN INVENTORY SYSTEM IN A HIGH SCHOOL LIBRARY INDICATED LOSSES OF 1 TO 6.48 PERCENT IN BOOK SECTIONS AND LOSSES OF 15.24 PERCENT AND 27.18 PERCENT IN PAPERBACK BOOKS AND MAGAZINES. TWO KITS PROVIDING IDEAS FOR LIBRARY THEFT PREVENTION ARE AVAILABLE FOR RENTAL OR PURCHASE. ALTHOUGH NONE ARE FOOLPROOF, THERE ARE A NUMBER OF THEFT DETECTION SYSTEMS AVAILABLE WHICH CAN SUBSTANTIALLY REDUCE LIBRARY LOSSES. A TOTAL SYSTEM FOR THEFT DETECTION WILL COST APPROXIMATELY $13,000 AND SHOULD REDUCE THEFTS BY AT LEAST 50 PERCENT. IN A LIBRARY WITH 450 TO 500 VOLUME LOSSES PER YEAR, SUCH A SYSTEM WOULD PAY FOR ITSELF IN ABOUT 3 YEARS. ONE SERIOUS PROBLEM WHICH SEEMS TO BE A BYPRODUCT OF DETECTION SYSTEM INSTALLATION IS AN INCREASE IN MUTILATION OF LIBRARY MATERIALS. INSTALLATION OF COPYING MACHINES DOES NOT APPEAR TO BE A SATISFACTORY SOLUTION TO THIS PROBLEM. REFERENCES ARE INCLUDED. (JAP)