Due to the spread of AIDS, distinguishing infectious from neoplastic lesions has become a major problem at many urban medical centers. It is not possible to separate CNS toxoplasmosis (Toxo) from CNS lymphoma on the basis of clinical, serologic or magnetic resonance (MRI) findings. The medical treatment for these two conditions is distinct and the currently utilized "empiric" therapy for Toxo can delay appropriate therapy. A non-invasive technique for identifying and separating these lesions would be helpful in the management of AIDS patients and is the goal of this proposal. We hypothesize that CNS Toxo can be differentiated from CNS lymphoma utilizing 1H NMR and postulate that these lesions can be distinguished by measuring a ratio of lactate to choline which will be 1 with Toxo and 1 with lymphoma. We further hypothesize that the Lac/Cho ratio will be useful in separating other brain infections such as cryptococcal granuloma from brain tumors. Finally, we speculate that loss of NAA and elevation of Cho seen with tumor, will normalize with successful treatment of these lesions. We will utilize 1H NMR to determine the in-vivo concentration of NAA creatinine (CR), Cho and Lac in patients with CNS Toxo, CNS tumors, and control subjects within the focal mass and compare it to normal appearing brain in the contralateral hemisphere 20 patients in each group will be remeasured after they have been successfully treated and the mass has regressed. In 20 patients with Toxo and 20 with lymphoma we will compare in-vivo 1H NMR measurements with in-vivo chemical measurements.