1. Synthesis of polyglutamate derivatives from radioactively labelled folic acid, folinic acid and methotrexate has been studied in human phytohaemagglutinin-transformed lymphocytes.

2. With labelled folic acid as the precursor, approximately 20% of the labelled cell folate was found in each of the mono-, tetra-, penta- and hexaglutamate peaks after 72 h of incubation. At earlier times, the amounts of labelled folate monoglutamate and lower folate polyglutamates were proportionately greater and the amount of labelled higher folate polyglutamates was lower. After only 4 h incubation over 80% of the intracellular labelled folate was still in the monoglutamate form.

3. With labelled folinic acid used as precursor, approximately 30% of labelled cell folate after 72 h incubation was in the form of folate pentaglutamate, with tri- and tetra-glutamate forming the other major peaks. The speed of formation of polyglutamate derivatives was substantially more rapid than from folic acid.

4. Methotrexate was found to decrease considerably the amount of folate polyglutamate formation from labelled folic acid in lymphocytes but nevertheless some di-, tri- and traces of tetra-glutamate were formed. On the other hand, methotrexate had no effect on folate polyglutamate formation with labelled folinic acid used as the precursor.

5. Polyglutamate derivatives of labelled methotrexate were formed by human lymphocytes with mean amounts of 4·5% of di-, 1·5% of tri-, 1·0% of tetra- and 0·7% of penta-glutamate derivatives being formed over the period 48–72 h of culture.

6. Pentaglutamate derivatives probably constitute the largest group of intracellular folates in human cells but a complex mixture exists.

7. The enzyme synthesizing polyglutamate derivatives of folate in human cells prefers a reduced folate as substrate but the requirement for a reduced form is not absolute. The nature of the reduced folate is uncertain but it is suggested on the basis of previous work that tetrahydrofolate rather than methylhydrofolate or any other reduced folate monoglutamate is the preferred substrate.

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