Culture of bone marrow-derived eosinophils in vitro. Bone marrow cells (2 × 10⁵/mL) from normal uninfected mice were cultured for up to 8 days in the presence of rIL-3, rIL-5, and rGM-CSF. (A) On the left, cultured cells were assayed for EPO activity at different times as described in Materials and Methods. On the right is the percentage of eosinophils counted on cytospin preparations with Wright Giemsa staining. Data are the mean ± standard error of at least three separate experiments. (B) Extraction of RNA from bone marrow cells was performed by using 1 × 10⁶ cells and RT-PCR of the MBP and β-actin were performed at different times.