We evaluated a new ready to use liquid assay for the homogeneous determination of HDL-cholesterol (HDL-C; Merck, Darmstadt, Germany) in comparison to phosphotungstic acid precipitation and a homogeneous assay, based on sulfated alpha-cyclodextrin and polyethylene glycol-modified enzymes (Roche Diagnostics/Boehringer Mannheim, Germany). The new liquid homogeneous HDL-C assay had inter-assay coefficients' of variation of less than 2.1%. The method is linear up to at least 3.11 mmol/I HDL-C, but even at 4.40 mmol/I the deviation from the expected value is less than 5%. Spinking experiments with low density lipoproteins and very low density lipoproteins proved that the new assay was specific for high density lipoproteins up to cholesterol associated with low density lipoproteins (LDL-C) and very low density lipoproteins (VLDL)-triglyceride concentrations of 18.13 and 22.60 mmol/l, respectively. Free fatty acids above 2mmol/l did not interfere. Icteric samples with bilirubin concentrations between 170 and 400 micromol/l did not show any systematic deviation compared to the precipitation procedure. In addition, serum hemoglobin concentrations up to 7.0 mmol/l and ascorbic acid up to 3000 micromol/l did not interfere with the HDL-C assay. An intermethod comparison including 120 samples revealed good agreement of the liquid HDL-C assay and the precipitation procedure (y = 0.943x + 0.074 mmol/l; r = 0.992). The new homogeneous HDL-C assay is thus precise, comparable and robust. Due to its ease of handling this assay will significantly facilitate attempts to include the differentiation between HDL-C and LDL-C in the routine screening for cardiovascular risk factors and in the monitoring of lipid lowering therapy.