Human immunodeficiency virus (HIV) was readily isolated by co-cultivation of patients' cells with phytohaemagglutinin-stimulated mononuclear cells from umbilical cord blood in 2 ml cultures in 24-well plates. Fluids from cultures of the MLA 144 cell line acted as an excellent source of interleukin-2, and promoted early replication of HIV in the primary cultures. Reverse transcriptase activity was commonly present at significant levels by 4-7 days. In contrast, recombinant IL-2 (recIL-2) did not promote early replication under these conditions. Adequate washing of the phytohaemagglutinin blasts was critical in this system, although others have reported it to be less important under other culture conditions. Cell concentrations and HIV: target cell ratios appeared not to play a major role in early outgrowth of virus. The particular sheep anti-alpha interferon tested resulted in a two-fold reduction in RT activity. Virus was readily transmitted in this simplified cheaper culture system.