FIG. 1.

Endpoint titration of the standard inoculum used for this study. Serially diluted brain homogenate of terminally sick CD-1 mice (30 μl) was injected i.c. into tga20 mice. Each circle represents one indicator mouse that succumbed to scrapie disease; each cross corresponds to a healthy animal. The 50% endpoint served to ascertain the titer of the 10% standard inoculum (42). The relationship y = 11.45 − 0.088x (y, log LD₅₀ per milliliter of homogenate; x, incubation time in days to terminal disease) was determined by linear regression (38).