The purpose of this study is to measure the levels of cytomegalovirus (CMV) in the blood of persons with HIV who are being screened for CMV retinitis, or who have been diagnosed with and are receiving treatment for CMV retinitis. This study is to determine the sensitivity and specificity of three different virological assays in predicting the development of CMV retinitis in AIDS patients with CD4 counts below 100/mm3. Secondly, the same three assays will be compared in terms of sensitivity and specificity in predicting treatment failures in subjects undergoing treatment for CMV retinitis. Asymptomatic subjects will be followed, using three different laboratory tests, for twelve months, with tests being performed at baseline and then every three months thereafter. Treated subjects will be followed at baseline, and then during induction and maintenance therapy for CMV retinitis until treatment failure is diagnosed clinically.