The CD11a/CD18 (LFA-1) leukocyte integrin receptor mediates homotypic and heterotypic leukocyte adhesion by binding to one of two defined ligands, ICAM-1 or 2, on the conjugate cell. In this study we investigated the molecular regulation of expression of the CD11a subunit during myeloid differentiation of HL-60 cells. Induction of monocyte/macrophage differentiation of HL-60 promyelocytic leukemia cells with PMA results in an increase in CD11a surface Ag expression and the acquisition of CD11a/CD18-mediated homotypic adherence. These changes are accompanied by a 40-fold increase in CD11a mRNA levels. Nuclear run-on transcription assays indicate that the increase in CD11a mRNA in PMA-induced HL-60 cells is not caused by an increase in CD11a RNA transcription. We assessed the posttranscriptional regulation of CD11a using two methods. By using actinomycin D to block RNA transcription, we demonstrate that the CD11a mRNA half-life in HL-60 cells is prolonged after PMA treatment. Inhibition of protein synthesis with cycloheximide also results in enhanced expression of CD11a mRNA in HL-60 cells without increasing CD11a transcription. These findings indicate that, in HL-60 cells induced with PMA to differentiate along the monocyte/macrophage pathway, CD11a expression is regulated primarily at the posttranscriptional level by a labile protein. Identification of the specific CD11a RNA sequences, and the proteins that bind to these sequences may provide insight into lineage commitment during human monocyte/macrophage differentiation.