Grapefruit seed extract. I just found some info online about it at holisticat.com:
GRAPEFRUIT SEED EXTRACT
Written by: Jean
Grapefruit seed extract, or GSE (not to be confused with grape seed extract) can be used as a natural antibiotic. It is an excellent remedy for skin diseases, external injuries, fungal infections, and internally, the diseases caused by parasites, bacteria, and viral or fungal infections (staph & strep bacteria, E-coli, salmonella, candida, herpes simplex & others).
However, use great care - it is very concentrated and the acid can burn tender skin. Always dilute it. Never use GSE full strength on or near your kitty, or on anything your kitty can lick.
For Internal Use (Never use full strength):
* The rule of thumb for gastrointestinal disorders, including bacterial, fungal, parasitic, or viral, is to use one drop of liquid concentrate or 10 milligrams of powder extract per 10 pounds (5 kg) of live weight.
* In acute cases, the dosage can be increased dramatically without ill-effect, but the wisest course is to increase frequency of dosage, rather than increase the size of dose by more than 2 or 3 times.
* As always, talk to your vet when any infection is present.
For External Use (Never Use full strength):
* Mix 30 to 40 drops of liquid extract in a quart of water, and spray on the infected area. If advisable, mix into a shampoo solution and leave on the skin or fur for a few minutes, then rinse well. This same dilution can be used as a general antiseptic for cages or litter boxes.
(By kind permission of Samuel E. Allen & Associates)