In order to elucidate the clinical significance of urinary apolipoprotein H (Apo H), otherwise known as beta 2-glycoprotein I, we first developed a non-competitive enzyme immunoassay (EIA) to quantify urinary Apo H levels. The measurable range of this assay was about 2 ng/ml to 100 ng/ml. The intra- and inter-assay coefficients of variation were 2.0 to 4.7% and 8.3 to 17.0%, respectively. These results indicated that this assay had high sensitivity and good reproducibility, and that it was useful for clinical study. We then determined urinary Apo H levels in 24 normal subjects and 36 diabetics using this assay. The mean urinary Apo H index (urinary Apo H/cr ratio), in 20 patients without proteinuria, who were regarded as patients without nephropathy, was 569 +/- 560 (X10(-3) mg/g.cr) and significantly higher than in normal subjects (252 +/- 147, p less than 0.01). The mean urinary Apo H index in 16 patients with overt proteinuria was 1,507 +/- 3,701 and also higher than in normal subjects (p less than 0.01). These results taken collectively indicate that urinary Apo H level using the EIA may be a new sensitive marker for detecting minor change of glomerular basement membrane in diabetics.