Nonproliferating cells (N cells) in the neural retina of embryonic chicks were estimated after isolating them in the 2C peak of a DNA distribution by exposure to the cell-cycle inhibitor ICRF 159. ICRF 159 inhibits cell division but not DNA synthesis, so proliferating cells can leave the 2C peak but not reenter it. Cells left in the 2C peak after exposure to ICRF 159 were assumed to be N cells. The effectiveness of ICRF 159 in inhibiting cell division but not inhibiting DNA synthesis was demonstrated in neural retinae from stage 17 embryos which showed no evidence of a 2C peak after 6-h exposure to ICRF 159 and which were thus shown to have no N cells. A test to detect escape from the cell division block in older embryos with an N cell population in the neural retina showed some escape after longer exposures to ICRF 159. The escape was suppressed by a second dose of ICRF 159, given some hours after the first.