The toxicity of whole, saturate, and aromatic hydrocarbon mixtures from flare pit and crude oil sources were evaluated using Lumbricus terrestris. Body burden analysis was used to analyze the intrinsic toxicity of the six hydrocarbon mixtures. The major fractions of the whole mixtures, the saturate, and aromatic fractions had different intrinsic toxicities; the aromatics were more toxic than the saturates. The toxicity of the saturate and aromatic fractions also differed between the mixtures. The flare saturate mixture was more toxic than the crude saturate mixture, while the crude aromatic mixture was more toxic than the flare aromatic mixture. The most dramatic difference in toxicity of the two sources was between the flare whole and crude whole mixtures. The crude whole mixture was very toxic; the toxicity of this mixture reflected the toxicity of the crude aromatic fraction. However, the flare whole mixture was not toxic, due to a lack of partitioning from the whole mixture into the lipid membrane of the exposed worms. This lack of partitioning appears to be related to the relatively high concentrations of asphaltenes and polar compounds in the flare pit whole mixture.