Purine analogues, 8-azaguanine (2.5 µg/ml); 6-thio- guanine (5 µg/ml); 6-mercaptopurine (50 µg/ml); 6- chloropurine (50 µg/ml); and 2,6-diaminopurine (50 µg/ml); inhibited the thymine growth response of Streptococcus faecalis Akt., a Methotrexate (Lederle trade name for 4-amino-N10-CH3-folic acid) resistant mutant of S. faecalis. However, these same concentrations of the purine analogues did not influence the thymine growth response of the parent strain of S. faecalis. The inhibitory effect of 8-azaguanine or of 6-thioguanine on S. faecalis Akt., was of a competitive nature and was reversed by 5 µg/ml each of adenine or guanine or 5 µg/ml each of the nucleosides or nucleotides of adenine or guanine. Pyrimidine analogs 2-thiouracil (100 µg/ml); dithiothymine (500 µg/ml); and isobarbituric acid (100 µg/ml); inhibited the thymine growth response of S. faecalis Akt., but did not influence the thymine growth response of S. faecalis. The inhibitory effect of isobarbituric acid on the thymine growth response was reversed by cytosine (50 µg/ml); uracil (50 µg/ml); deoxycytidine (50 µg/ml); thymine (50 µg/ml); or thymidine (50 µg/ml). The results suggest that the S. faecalis Akt. has acquired de novo purine and pyrimidine biosynthetic pathways and is therefore inhibited by the analogs at lower concentrations. It may be speculated that clinical resistance to methotrexate may be associated with the acquisition of de novo purine biosynthetic capabilities in methotrexate-resistant cells and this opens a new avenue of attack on methotrexate-resi