Labeled ribonucleic acid (RNA) complementary to Agrobacterium tumefaciens DNA and PS8 bacteriophage DNA (cRNA) were used in a systematic study of the sensitivity of cRNA/deoxyribonucleic acid (DNA)-filter hybridization for detection of small amounts of phage or bacterial DNA immobilized on filters. A. tumefaciens cRNA of specific activity 10(6) to 2 x 10(6) counts per min per mug reacted to a significant extent when the DNA-filter contained 1% A. tumefaciens DNA in a salmon DNA background, but 0.1% A. tumefaciens DNA was not detectable. PS8 phage cRNA of the same specific activity reacted to a significant extent when the DNA-filter contained as little as 0.01% PS8 DNA in a salmon DNA background. Both kinds of cRNA were found to bind to tobacco crown gall tumor DNA-filters. Similar reaction was found with control normal callus DNA-filters but not with tobacco seedling DNA-filters. The "hybrids" formed by cRNA with normal callus and tumor DNA-filters had low thermal stability. Attempts to purify the tumor and normal callus DNA prior to immobilization on the filter resulted in elimination of this spurious binding. No evidence was found for bacterial or phage DNA in crown gall tumor DNA.