The aim of this study was to investigate whether the tumor‐inhibiting activity present in the rat pineal gland undergoes seasonal fluctuations as do other pineal substances. Crude ethanol extracts of rat pineal glands were tested for tumor‐inhibiting activity in an in vitro microbioassay using human erythroleukemia cells which could not be inhibited by melatonin. Highest activity was detected in summer and least inhibition and even stimulation were observed in winter. There were no differences in activity between animals of different age, sex, or strain. Therefore, season seems to be the factor that exerts the most important influence on the content of tumor‐inhibiting activity in the rat pineal gland. Correlation with seasonality in the occurrence of cancer is disc
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