Light-synchronized periodicity analysis reveals a susceptibility rhythm in C mice to E. coli endotoxin. Susceptibility varies predictably and significantly along the 24-hour time scale. A dose of endotoxin which is compatible with survival of most animals when given during middle of daily dark period is highly lethal when it is given 8-12 hours earlier or later. LD₅₀ also was determined for 2 samples from identical batch of endotoxin tested 12 h apart on separate groups of comparable C mice kept under conditions standardized for periodicity analysis: Differences in potency significant at 1% were seen at 2 dose levels common to both assays, the computed “potency ratio” being 3.22.