1. The effects of 3-methylcholanthrene pretreatment on both N- and ring hydroxylation of 2-acetamidofluorene by microsomal preparations from various well-differentiated and poorly differentiated hepatomas, primary tumours and their host livers and kidneys were studied. 2. Well-differentiated Morris hepatomas 5123C, 5123D, 5123CTC and 7800 and their host livers had low hydroxylating activity. Pretreatment with 3-methylcholanthrene caused a several-fold increase in both N- and ring hydroxylation in the host livers whereas in all tumours except 5123CTC it caused a many-fold increase only in ring hydroxylation. 5123CTC tumour in addition showed a fourfold increase in N-hydroxylating activity. 3. Hydroxylating activities of poorly differentiated Morris hepatoma 7288CTC and Novikoff hepatoma were low and they could not be altered by 3-methylcholanthrene pretreatment. 4. Primary hepatomas produced by administration of 4-dimethylamino-3′-methylazobenzene could be stimulated to some extent on 3-methylcholanthrene pretreatment; however, primary mammary tumour produced by administration of 3-methylcholanthrene was not responsive to 3-methycholanthrene pretreatment. 5. Like host livers, kidneys of tumour-bearing animals could also be stimulated to some extent by 3-methylcholanthrene pretreatment.

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