Nucleocidin shows inhibition occurs at low concentrations, however, it is not possible to obtain complete inhibition at 10-3 M. Essentially complete inhibition is obtained at 10-3 M puromycin, and only at the lowest concentrations is any lag detectable. It seems that Nucleocidin exhibits the pattern typical of many antibiotics, i.e. it inhibits incorporation at a stage subsequent to the formation of aminoacyl-S-RNA. An appreciable lag occurs before any inhibition by Nucleocidin is detectable, and the length of this lag period varies inversely with the concentration of Nucleocidin. Similar results are obtained when Nucleocidin is added to the cell-free system from reticulocytes .