SB203580 inhibits p38 MAPK activity and retards apoptosis of HCD-57 cells in the absence of EPO. Cells were pretreated with 10 μmol/L SB203580 for 4 hours in normal growth medium before starvation with EPO-free medium supplemented with the same concentration of the inhibitor. The inset in the top panel shows p38 MAP kinase activity after 2 days of EPO starvation, as determined by using ATF-2 as a substrate. Apoptosis was determined by flow cytometric assays after 1 to 4 days of EPO withdrawal (top panel) and by fluorescent cell staining after 2 days (bottom panel). The photograph was taken with 400× magnification. The stock solution of SB203580 was made in water. Open bars and bottom, left photo, EPO-containing medium; reverse-slashed bars and the bottom, middle photo, EPO-free medium; and slashed bars and the bottom, right photo, EPO- free medium plus 10 μmol/L SB203580.