 Medication adherence will be measured as: rates, by interview and documentation on tenofovir adherence card, of participants taking at least six 86 of seven daily doses of study drug each of the four weeks preceding the monthly study visit. Differences in virologic and immunologic responses to HIV infection among tenofovir and placebo recipients will be measured by: plasma viral load, measured by quantitative RNA PCR, a predictor of clinical progression of HIV disease; 14 CD4 cell counts will be measured by flow cytometry. Rates and nature of HIV antiretroviral genotypic and phenotypic resistance will be measured. Genetic characteristics of infecting HIV viruses including DNA sequence analysis and antibody binding studies will be conducted. 