Toxicity of PEI-mediated gene transfer is dose-dependent. On the fifth day in vitro, sympathetic neurons were counted on an inverted phase-contrast microscope (20x) and the total number in each well was estimated from a random sampling of 20 microscopic fields per well. Cells were then transfected with 0.2: 1, 0.5: 2.5, or 1: 5 μg DNA: μg PEI. Three days later, cells were fixed and counted again, pairing the post-transfection cell number in a well with its corresponding pre-transfection number. Data are expressed as the mean % cells surviving transfection of three separate wells ± SEM.