Acyltryptophols reversibly inhibit the uptake of thymidine after phytohaemagglutinin transformation of human lymphocytes.

1. Lymphocytes were stimulated for 72 hours with phytohaemagglutinin (PHA), [3H]-thymidine was added and, after a further 4 hours incubation, its uptake into DNA was measured. 2. Addition of acetylmethoxytryptophol (aML) and other acyl tryptophols simultaneously with the PHA caused an immediately reversible, dose-dependent reduction in uptake of thymidine into the DNA. 3. Similar additions one hour before the addition of thymidine had identical, immediately-reversible effects. 4. Similar addition up to about 30 min after the addition of thymidine showed a reduced but still dose-dependent effect on the amount of DNA radioactivity. 5. It is concluded that aML, and related acyltryptophols, act by inhibition of thymidine uptake into the lymphocyte, in which case the compounds probably dissolve in the lipid bilayer of the membrane and act as channel blockers.