I am opening an excel file like this:
Excel.Application xlApp; Excel.Workbook xlWorkBook; Excel.Worksheet xlWorkSheet; Excel.Range range; string str; int rC
Once you have a reference to the worksheet say
for(int i = 1; i <=100; i++) { if(!worksheet.Cells[i,1].Contains("SomeString")) { ((Range)worksheet.Rows[i]).Delete(shiftDirection) } }
where shiftDirection see here: Range.Delete method
You may have to cast the Cell's content to a string.