The accuracy of a commercially available whole blood glutaraldehyde clot test in the detection of failure of passive transfer (serum immunoglobulin [Ig]G1 < 1,000 mg/dL) in neonatal calves was evaluated. Serum samples were obtained from 242 calves ranging in age from 1 to 8 days, and comparisons were made with serum lgG1 concentrations determined by radial immunodiffusion. Both the sensitivity and specificity of the currently marketed whole blood glutaraldehyde clot test are inadequate for routine diagnostic use. Concerns regarding test sensitivity are the most problematic. Sensitivity varied from 0.41 to 0.00, depending on the choice of test endpoint. Specificity varied from 0.85 to 1.00, depending on the choice of test endpoint. Regression analysis demonstrated that the relationship between serum lgG1 concentration and the glutaraldehyde clot results, although significant (P< .10), was of negligible biological relevance (r2= .034). J Vet Intern Med 1996; 10:82–84. Copyright © 1996 by the American College of Veterinary Internal Medicine.