To define the optimal conditions for human immunodeficiency virus (HIV) detection in microcultures, experiments were conducted with different ratios of patient and donor peripheral blood mononuclear cells (PBMCs). Donor/patient PBMC ratios ranged from 1:1 to 1:125. Optimal results were obtained when 1,500,000 donor cells were cocultured with equal or smaller quantities of patient PBMCs. Thus, virologic endpoints could be achieved by diluting patient cells. Smaller numbers of donor cells, with or without larger numbers of patients cells, resulted in lower rates of HIV isolation. Similarly, the direct stimulation of patient PBMCs with phytohemagglutinin without the addition of normal donor cells lowered the sensitivity of the assay significantly. We suggest that a microculture procedure using a fixed quantity of donor cells with different dilutions of patient cells may be useful for monitoring changing HIV levels during antiviral therapy.