Erythroid burst-forming units (BFU-E) from mouse bone marrow were grown for 7 days in agar or serum-free methylcellulose cultures in the presence or absence of erythropoietin (Ep) and/or interleukin 3 (IL-3). It was found that IL-3, even in the absence of serum and detectable Ep, was able to stimulate the full development of many erythroid bursts. This IL-3 effect was cell-dose dependent and did not appear to correlate with Ep dose. Spontaneous bursts and those stimulated by Ep only were rare and when seen were very small relative to those produced by IL-3 or IL-3 plus Ep. When addition of IL-3 or Ep to 7-day cultures was delayed, IL-3 but not Ep was shown to maintain BFU-E. No evidence was found by radioimmunoassay that Ep was produced or released in 7-day, "serum-free" cultures of bone marrow nor was Ep activity detected in culture media except those to which it had been added deliberately.