Growth of the Walker 256 tumor in vigorously exercised male Wistar rats was compared with that of control animals confined in small, individual cages, permitting a minimum of movement. In each instance the weight of the tumor in the control group significantly exceeded that in the exercised group. In several instances there was complete tumor regression in the exercised animals.

Rat muscles were suspended in a 0.85 per cent NaCl bath and electrically stimulated to “fatigue.” An extract of this bath (F-Substance) inhibited the growth of the Walker 256 and Murphy lymphosarcoma when injected either at the tumor site or on the opposite side. An extract prepared in a similar manner from nonfatigued muscle was found to be ineffective.

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Supported in part by the Damon Runyon Memorial Fund for Cancer Research, Inc. (DRG Grant 618).

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