Human wart virus was isolated from plantar warts. After extraction of its DNA, component I was transcribed into radioactive complementary RNA (cRNA) with the aid of Escherichia coli RNA polymerase. The resulting cRNA annealed specifically to wart viral DNA and was used as a probe for the detection of wart viral DNA in human warts, condylomata acuminata, laryngeal papillomas and some malignant human tumors. High concentrations of hybridizing DNA were found in plantar warts. Verrucae vulgares annealed to a considerably lower extent only, indicating that very few genome equivalents were present in those papilloma cells. Some verrucae vulgares were found to be completely negative in this test. Condylomata acuminata, as well as laryngeal papillomas and all malignant tumors tested, did not hybridize with wart viral cRNA.