validate a value from a textbox column in dataGrid

I have a datagrid which has three columns and one is a textBox templateColumn Quantity. I want to validate the value entered by the user in this column... ...is there a way I can validate the value. the other two columns get their value from the Xml file..so the textbox column is the only one where a user enter the value.

Thanks for your help
Neelayu can use validator control. if i need to do that, i just open the page in HTML format and type the necessary code with ControlToValidate property set to the control for which the validator is to be used.