To assess whether the Lowry-tetramethylurea method for measuring apolipoprotein B-100 (apo-B) in very low density lipoprotein (VLDL) could be replaced by direct assay for VLDL apo-B using a highly practicable immunological method. Seventy five fasting blood samples were collected from patients attending the lipid clinic at this hospital. Plasma was separated immediately and VLDL isolated by preparative ultra-centrifugation at solution density 0.93-1.006 kg/l. Apo-B was precipitated from aliquot of the VLDL fraction using the tetramethylurea (TMU) technique and protein mass determined by the Lowry method (LM); mean apo-B 83.02 mu g/ml (SD 74.85). Apo-B was also measured in VLDL using direct immunoturbidimetry on the Cobas-Fara analyser; mean apo-B 82.32 mu g/ml (SD 72.88). There was a very close correlation between methods (immunoturbidimetry = 0.94 LM + 3.95, r = 0.97, p <0.001). The mean difference between methods (constant error) was small (0.70 mu g/ml) and not significant (p = 0.742). Random error was 13.01 mu g/ml by analysis of variance.It is concluded that immunoturbidimetry, a more rapid and convenient test, may replace the LM and TMU techniques for measuring VLDL apo-B concentration and that this method could be applied to research studies requiring analysis of large numbers of samples.