It appears likely that there will be a delay and possibly a reduction in the amount of influenza vaccine available for the 2000-2001 influenza season. One possible way of increasing the availability of influenza vaccine for this year is to use a half-dose in healthy adults. The objective of this study is to determine if the immune system responds to a half-dose the same way it responds to a whole dose. This study will use the currently approved inactivated influenza vaccine in healthy adults ages 18 to 49 years old.