> Is that law written in Leivticus? I know the 11th chapter lists the unclesn
> meats that shouldn't be eaten.  Where is the health law about not mixing
> meat and dairy? I'd like to Mark that in my book. Sue Gallant

Relevant Paragraph from http://www.mechon-mamre.org/jewfaq/kashrut.htm:

Separation of Meat and Dairy

On three separate occasions, the Torah tells us not to "boil a kid in its mother's milk" (Exodus 23,19; Exodus 34,26; Deuteronomy 14,21).  The Oral Torah explains that this passage prohibits eating meat and dairy together.  The rabbis extended this prohibition to include not eating milk and poultry together.  It is, however, permissible to eat fish and dairy together, and it is quite common.  It is also permissible to eat dairy and eggs together.  According to some views, it is not permissible to eat meat and fish together, but we are not certain of the reason for that restriction (it has been attributed to medical opinion in the Middle Ages, for example).

This separation includes not only the foods themselves, but the utensils, pots and pans with which they are cooked, the plates and flatware from which they are eaten, the dishwashers or dishpans in which they are cleaned, and the towels on which they are dried.  A kosher household will have at least two sets of pots, pans, and dishes:  one for meat and one for dairy.  See Utensils below for more details.

One must wait a significant amount of time between eating meat and dairy.  Opinions differ, and vary from one or two to six hours.  This is because fatty residues and meat particles tend to cling to the mouth.  From dairy to meat, however, one need only rinse one's mouth and eat a neutral solid like bread, unless the dairy product in question is also of a type that tends to stick in the mouth.
The Yiddish words fleishig (meat), milchig (dairy), and pareve (neutral) are commonly used to describe food or utensils that fall into one of those categories.

Note that even the smallest quantity of dairy (or meat) in something renders it entirely dairy (or meat) for purposes of kashrut.  For example, most margarines are dairy for kosher purposes, because they contain a small quantity of whey or other dairy products to give it a dairy-like taste.  Animal fat is considered meat for purposes of kashrut.  You should read the ingredients very carefully, even if the product is kosher-certified.