How to make particular cell in datagrid readonly

Question

Hi, I populated the employee payroll data (name, department, noofdays, salary, deduction) into datagrid. now, the situation is one can change the value of the deduction column, if noofdays column value is > 15. I need to make few cells in the complete grid readonly, i tried setting the column property to readonly which makes the complete column so. Your help is highly appreciated. Ram