In an extension of the experimental studies recorded in an associated paper; attempts were made to isolate and characterize the constituent of guinea pig serum responsible for inducing regression of transplanted lymphomas in vivo. The active material was precipitated readily from the whole serum, along with some of the globulins, by means of ammonium sulfate in concentrations of 2.0 molar or greater; it withstood heating at 56°C. for 20 or 30 minutes, but was inactivated upon heating at 66°C. for similar periods; it was completely inactivated by chymotrypsin in concentrations of 1 or 2 mg./cc. during 6 hours at 37°C. Furthermore, the inhibitory effects of small amounts of the guinea pig serum in vivo were enhanced upon admixture with immune sera prepared by injecting the lymphosarcoma cells into rabbits. The facts as a whole suggest that the active material is a protein, and that it may be one or another of the components of complement; yet they do not suffice to establish its identity.

Microscopic studies showed that the cells of subcutaneous lymphomas rapidly died and were resorbed following injections of relatively large amounts of guinea pig serum intraperitoneally into mice carrying them, while similar changes followed more gradually after repeated injections of smaller amounts of guinea pig serum. No changes referable to the guinea pig serum were seen in the normal tissues or organs of mice receiving it.

Mouse lymphoma cells, suspended artificially as individuals in a physiological saline solution, regularly remained viable following incubation in vitro in mixture with guinea pig serum during 6 hours at 37°C. The finding provides strong evidence that the regression of lymphomas that follows injection of guinea pig serum in vivo is brought about through some reaction in which the guinea pig serum and the host both participate.

Some of the implications of the findings are discussed.

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