The potential relationships between chlorinated hydrocarbon contamination in human serum and red/white blood cell profiles were investigated by multivariate techniques to assess the cellular response patterns to high and low organochlorine levels in the serum. Twenty-three healthy control subjects and fourteen patients with unexplained and persistent fatigue were divided on the basis of (a) high or low total organochlorine content, (b) high or low DDE (1,1-dichloro-2,2-bis(p-chlorophenyl) ethene) content, and (c) high or low HCB (hexachlorobenzene) content. Discriminant function analysis revealed that the groups with high organochlorine content had significantly different red/white blood cell profiles compared with the low organochlorine groups ((a) P < 0.017, (b) P < 0.015, and (c) P < 0.0002). As a variable, the percentage of neutrophils was the most important discriminant parameter for differentiating between the high and low total organochlorine groups. Thirteen of the fourteen fatigued patients were characterized as "high total organocholorine content" (P < 0.04). The red cell distribution width was elevated in the high DDE group (P < 0.04) and was the most important discriminant parameter for differentiating between the high and low DDE groups. The percentage of eosinophils and the hemoglobin content were both reduced in the high HCB group (P < 0.009,P < 0.003, respectively) and the percentage of eosinophils was the most important discriminant parameter for differentiating between the high and low HCB groups. Those patients with unexplained and persistent fatigue had significantly higher levels of DDE compared with the controls and had different specific blood cell responses to organochlorines compared with control subjects.