Class initialization Property vs Field

I was wondering what's the best practice for class initialization.

One can write:

private myClass mc = new myClass();

Or:

private myClass mc { get; set; }

public Foo()
{
    mc = new myClass();
}

I always tend to use it as a field. Are there any downsides for using a class as a property ? I did some searching on google but none of the results gave me a good answer for this specific question.