Abstract
How genetic experiments are being used to probe the nature of the genetic control of combining sites of antibodies is described especially to see whether each such site is controlled by stable genes transmitted in the gene line. There are hints of such stable control, but more often genetic experiments are revealing steps other than combining site formation, like the one in guinea-pigs blocking response to determinants carried on a poly-L-lysine carrier and in mice to determinants carried on the influenza virus.