In this study groups of the epigeic earthworm species Eisenia fetida were exposed experimentally for different periods to sublethal concentrations of Pb(NO3)2. The lysosomal membrane fragility response of coelomocytes was used as a biomarker to explore the cytotoxity of Pb by determining the neutral red retention times (NRRTs) and correlating them with different body loads of Pb. Previously unexposed worms exposed to different low concentrations of Pb(NO3)2 for 4 weeks demonstrated a clear dose-response relationship between NRRTs and body burdens of Pb. The NRRTs of growing worms with a previous (ancestral) exposure history, exposed to an elevated concentration of Pb and measured over time, fluctuated around a mean of 41 min and no dose-related relationship could be demonstrated. Worms with a long-term history of exposure to Pb of more than 36 months had a mean body burden of 177.5 +/- 61.5 mg/kg Pb and a mean NRRT of 57.8 +/- 3.1 min, which was significantly lower than that of uncontaminated, control worms with a mean time of 61.9 +/- 3.4 min and a mean body burden of 86.3 +/- 47.2 mg/kg. No significant differences were found between weight changes of "clean" and preexposed worms when they were experimentally exposed to sublethal concentrations of Pb. This study provided evidence that the lysosomal biomarker response for Pb may not persist at the same level in worms with a previous history of Pb exposure, possibly making it less sensitive as a tool for comparing responses to Pb in field populations of earthworms.