A clinical test for the presence of antibodies in a serum, based on whether an antigen and complement, when put together with some red blood cells, are able to destroy them.
If the red blood cells are destroyed, then antibodies were not present to prevent it. If the red blood cells are not destroyed, then antibodies were present to combine with the antigen and bind the complement, making them unable to attack the red blood cells.
This is often simply referred to as a "CF test".
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.