DDT was found to be absorbed very slowly from acetone or benzene solutions when applied topically to susceptible and DDT-resistant human body lice. The resistant strain could tolerate larger quantities of applied DDT without any appreciably harmful effect. The feeding of lice through dissected chick skins on citrated human blood containing various concentrations of radioactive DDT showed that the resistant strain could tolerate 100 ppm of DDT in the blood whereas 10 ppm caused 53 per cent mortality in the susceptible strain.
Only resistant lice metabolized DDT in vivo to a relatively nontoxic material unextractable with common organic solvents but partly extractable with methanol. Determination of the metabolite by the Schechter-Haller method yielded a nitrated product which was removed from solution by dilute alkali, unlike DDT, DDE, or DDA.
Both susceptible and DDT-resistant lice metabolized DDT in vitro at approximately the same rate. The nontoxic, water-soluble metabolite gave a red-colored complex in the Schechter-Haller procedure and had solubility characteristics similar to that obtained in vivo.
It appears that an intrinsic biochemical factor which, presumably, interferes with the detoxifying mechanism, might be operative in the susceptible strain. This interference is manifested in vivo but not in vitro.