Extract of rat, rabbit or bull testicle prevents or retards the growth of a rabbit tumor when a mixture of the extract and a tumor cell suspension is inoculated intradermally. Similar mixtures, made with normal rabbit serum instead of testicle extract, give rise to tumors which grow with unusual rapidity. The results are the opposite of those obtained with pathogenic bacteria or filtrable viruses which are enhanced by testicle extract and generally inhibited by normal serum.

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