The response of cells of the megakaryocytic lineage to interleukin 6 (IL6), a cytokine with multiple biological activities, was studied by adding the factor to human bone marrow (BM) cultures. IL6 alone had no effect on megakaryocytic colony formation in methylcellulose; however, in the presence of maximally stimulating concentrations of IL 3, almost a twofold increment in colony formation was observed. Tritiated thymidine suicide studies of BM incubated for 2 h with growth factors showed that almost one-half of megakaryocytic progenitors (CFU-Mk) preincubated with IL3 or IL3 plus IL6 were in S phase, whereas BM incubated with IL6 alone was similar to control (approximately 24% of CFU-Mk in S phase). When greater than or equal to 1 ng/ml of IL6 was added to liquid suspension cultures of BM, the size of individual megakaryocytes was significantly augmented compared with that seen in control cultures. Moreover, the DNA content of megakaryocytes grown in the presence of IL6 was increased (modal ploidy 16N) compared with cultures grown with IL3 (modal ploidy 8N). To determine if the effect of IL6 could be direct rather than mediated via accessory BM cells, the factor was added to cultures of isolated single megakaryocytes. Seventy-one percent of cells increased in diameter in the presence of IL6, while only 27% increased in size in the absence of the factor. The data show that IL6 is a direct maturation factor for human megakaryocytes, promoting increments in size and ploidy of these cells.