using System;

using System.Linq;

using System.Xml;

using System.Xml.Linq;

public class Program

{

    public static void Main()

    {

        string yourXMLString = @"<T24.MESSAGE>

<TRANSACTION.TYPE>CHEQUE.ON.THEM.AUTH</TRANSACTION.TYPE>

<TRANSACTION.ID>FT14273PKQ14</TRANSACTION.ID>

</T24.MESSAGE>";

        XDocument xDoc = XDocument.Parse(yourXMLString);

​

        var res = xDoc.Descendants("T24.MESSAGE")

                      .Elements("TRANSACTION.TYPE").FirstOrDefault();

        Console.WriteLine(res);

    }

}