Limiting dilution titration of RML scrapie in tg44 and C57 mice. Serial ten-fold dilutions of RML scrapie were inoculated i.c. in a volume of 50 μl into either tg44 (squares) or C57 (circles) mice. Each point indicates the incubation period for one mouse; in cases of overlap the symbols have been separated to facilitate observation. Solid symbols indicate mice that had clinical signs and detectable PrPres in their brains. Open symbols indicate mice that did not develop disease or detectable levels of PrPres in brain. Using the data shown, the calculated titers for RML scrapie were: tg44 mice, 108.1 ID50/gram of brain and C57 mice,107.9 ID50/gram of brain.