This article is from the the official alt.spleen FAQ, by Andrew Stellman with numerous contributions by others.
Andrew says: NOTE: PLEASE DO NOT SEND ANY MAIL TO ME REQUESTING MEDICAL ADVICE! I AM NOT A DOCTOR, AND WILL NOT GIVE YOU ANY USEFUL INFORMATION.
In general, .20 cc's of adrenaline for every ten grams of spleen. For example, if your spleen weighs 150 grams, you should administer a 3 cc injection. The injection should be administered with a four-inch needle. Remember, you have to come down in one quick motion into the center of the spleen, so as to penetrate the tough exterior muscular wall. Note that we are talking about dry weight, not post-meal weight. (See above section on spleens changing sizes.)
DISCLAIMER: Previously, an incorrect dosage was described above. Thanks to Jen (email@example.com) for pointing it out. The FAQ maintainer takes no responsibility for overdoses or cases of adrenalyne shock.