A Purine and Pyrimidine Requirement for Paramecium multimicronucleatum*

SUMMARY. After removal of nucleic acids, by enzymatic digestion and dialysis, from an unidentified component of the medium, it was possible to show specific purine and pyrimidine requirements for Paramecium multimicronudeatum. The purine requirement was satisfied by guanosine, deoxyguanosine, guanylic acid or deoxyguanylic acid, but not by any of the adenine derivatives. Cytidine, deoxycytidine, cytidylic acid, deoxycytidylic, uridine, uridylic acid or deoxyuridine would serve as the pyrimidine source, whereas thymidine and thymine would not. The free bases guanine, cytosine and uracil did not replace their respective pentose derivatives. Of the active compounds, the pentonucleosides were more active than the pentonucleotides. Inbsine, xanthine, 2,6‐diamino‐purine, 5‐methylcytosine, and orotic acid did not support growth.