We investigated the lipoproteins and apoproteins in human serum and pleural effusions of different origin: transudates, inflammatory exudates, and malignant exudates. Transudates had a low cholesterol content of 35 +/- 12 mg/dl (mean +/- SD) because of low levels of low-density lipoprotein (LDL) cholesterol--representing 16% of serum levels--whereas inflammatory exudates (cholesterol 92 +/- 26 mg/dl) and malignant exudates (cholesterol 86 +/- 6 mg/dl) exhibited high levels of LDL, with 67% and 69% of serum levels. Apolipoprotein (apo) B level corresponded with LDL and presented with multiple split-products in sodium dodecyl sulfate-polyacrylamide gel electrophoresis in exudative effusions. LDL levels in effusions correlated with serum levels in exudates but did not correlate with those in transudates. In contrast, lipoprotein(a) appeared in all effusions from patients with detectable serum levels. The isoforms were similar as demonstrated by immunoblotting. Differences were found in the composition of the high-density lipoprotein (HDL) fraction: transudates had cholesterol-rich HDL when compared with serum. HDL particles of malignant exudates were poor in cholesterol, and isoelectric focusing demonstrated more sialized apolipoprotein E. A strongly abnormal HDL level with accumulation of cholesterol was found in a long-standing tuberculous effusion. In conclusion, cholesterol in acute effusions is bound to lipoproteins and derived from the blood. The difference in total cholesterol levels between transudates and exudates is based on the lack of LDL in transudates. Transudates show the lipoprotein characteristics of interstitial fluid. Alterations of lipoproteins occur in chronic inflammation and in malignancy with possible de novo synthesis of apolipoprotein E by tumor cells. Lipoprotein(a) accumulates independently from LDL in the pleural space, a finding that supports the view that the physiologic function of lipoprotein(a) is located in the interstitial space.