Carcinogenic substances in human tissues.

Some of the problems presented by the carcinogenic substances that have been extracted from human tissues will be solved only when these compounds have been chemically identified and assayed. At the present time very difficult questions have been raised by the fact that the substances have been found in the tissues from some patients dead of cancer but not in others, sample of 67 human livers, 1 in 5 contain detectable carcinogens. It would not be very satisfactory to "explain" on a fortunate chance encounter of "1 in 5" why Schabad (9, 10), who was the first to open this line of investigation, found a benzene extract of a noncancerous liver to be inactive while the benzene extract of a can-