Margins appear to exist

Hi

I have defined a definition for table styles, which works perfectly except that it appears to be giving margins (space between cells and between the cells and the edge of the table).

I know it's margins (and not borders), because changing the background colour of the table changes the colour of the space which appears.

However, I have set margins to none:

.maintable
{
	left: 0px;
	width: 750px;
	position: absolute;
	top: 117px;
	background: none;
	margin: 0px;
}

(It appears similarly in IE and Mz). Am I going made or just missing something so blindingly obvious I will kick myself?

David