The hypothesis that the properdin system may play a role in the natural resistance of animals to transplantable tumors was investigated by the use of the transplantable carcinoma of the human colon HR132 in normally nonsusceptible weanling Wistar rats.

In a duplication of a previous experiment, it was again shown that the rats can be conditioned to the human tumor by intravenous injection of appropriate doses of zymosan but that such conditioning is not as effective as that produced by exposure to 300 r total-body irradiation.

The properdin levels of sera from animals so conditioned showed no constant relationship to tumor “takes” inasmuch as some were slightly depressed, others were normal, and others still were elevated.

The administration of human properdin to irradiated animals increased the properdin titers of their sera to almost threefold, yet it did not restore their resistance to implants of the human tumor.

It is thus concluded that, under the conditions of the experiments, the natural resistance of weanling Wistar rats to the transplantable carcinoma of the human colon HR132 is not mediated through the properdin system.

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