SUMMARYJoint culture of two incompatible plasmodia end in the loss of one phenotype. The survivor can be predicted by comparing the cytotoxic potential of the two phenotypes because the survivor should have a greater total toxicity potential. Also, the reliability of the predictions should increase with increasingly higher differences in toxicity potential. The elimination of one (or more) plasmodial-incompatibility phenotypes from culture is viewed as one more manifestation of a self-nonself recognition system.