IMTH has been reported to degrade DNA in solution. Therefore steps of protein biosynthesis in vivo and in vitro were investigated after pretreating rats in vivo with IMTH ( 3 × 40 mg ip, every 2nd day). Our results are as follows: 1. In the isolated perfused rat liver, protein biosynthesis comes nearly to a full stop, but there is no clear evidence whether IMTH acts in vivo on DNA or the biosynthesis of m-RNA, r-RNA or s-RNA. The number of polysomes seems to be normal; ribosomes can accept polyuridylic acid. 2. The ability of pH- 5-enzymes to load s-RNA and the capacity of s-RNA to accept labelled amino acids is normal; loaded s-RNA of pretreated animals can enter polysomes of control rats. 3. It has been shown that incorporation of C 14 in vitro is lower than normal when polysomes of pretreated animals are used; this effect is coupled with an inhibited polysome breakdown. 4. Labelled IMTH is bound in small amounts to the microsomal fraction, dependent upon time, temperature and concentration, but barely upon pH. 5. Small effects on nucleic acids or additional effects on mitochondria are not excluded by these experiments.