Light-synchronized periodicity analysis reveals a susceptibility rhythm in C mice to Escherichia coli endotoxin. Susceptibility varies predictably and significantly along the 24-hr. 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 hrs. earlier or later. LD50 also was determined for 2 samples from identical batch of endotoxin tested 12 hr. 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.