The present study has investigated the effect of PMA, an inducer of monocyte differentiation, on HIV expression in a chronically infected promonocyte clone. After acute HIV infection of U937 cells, clones that constitutively expressed varying levels of HIV were isolated by limiting dilution. One clone (U1) produced low levels of HIV but was found to increase its production 20-fold after PMA induction, as detected by reverse transcriptase or A capture. Further characterization of U1 indicated that PMA could induce cellular differentiation and maturation in the clone similar to that in uninfected U937 cells. In addition, functional studies revealed that superoxide anion production from the U1 clone was not different from that of uninfected U937 cells. Electron microscopic studies of U1 indicated that PMA induced endocytotic vesicles containing many HIV particles. These studies provide a model at the clonal level to 1) examine latency or chronicity of HIV infection in monocytes and 2) delineate the signals required for conversion to high level viral expression.