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An evaluation of spurious, nonneutron, background counts from BF3 proportional counters has been made. This evaluation indicates that the source of ionization events within the counters is from alpha particles originating in natural uranium series bulk contamination of the counter materials. This leads to an irreducible minimum background counting rate in the absence of rfeutrons which is characteristic of each individual counter. Measurements on a number of commercial counters gave backgrounds of from 0.005 to 0.05 cpm per square inch of inner surface. Careful materials selection and special processing techniques should reduce this source of background considerably.