Summary: The dose of ovalbumin per individual required to induce fatal anaphylactic sensitivity in guinea pigs within the age-limits of 2 to 20 weeks was unaffected by differences in age or body-weight. Old animals, 1 to 3 years of age, were less susceptible to sensitization and required about 20 times as much antigen to induce fatal sensitivity. Groups of guinea pigs sensitized at 2, 4, and 7 weeks of age and challenged at 5, 7, and 10 weeks of age, respectively, required essentially the same dose of antigen per individual to elicit fatal shock. Animals sensitized at 17 weeks of age and challenged at 20 weeks required about twice as much antigen to elicit fatal shock. Male and female guinea pigs were equally susceptible to anaphylaxis.