Objective To evaluate the analytical performance of the testosterone assay performed on the TECHNICON IMMUNO 1 system from Bayer Corporation.Results Very good within-run and between-day CVs were obtained (less than 5%) for testosterone concentrations varying from 3.24 to 67.0 nmol/L. Within-run CVs of 8.4 and 12.9% and between-day CVs of 4.9 and 11.4% were obtained for testosterone concentrations of 1.77 and 0.66 nmol/L. The minimum detectable concentration was estimated at 0.10 nmol/L. The assay linearity proved excellent. A good correlation between the TECHNICON IMMUNO 1 and the Diagnostic Products Corporation (DPC) assays was observed with different categories of serum samples (IMMUNO 1 = 1.11*DPC-0.32, r = 0.989, Syx = 2.07 nmol/L, n = 347). We determined the reference ranges at 0.3-3.2 nmol/L for females, and at 7.9-36.3 nmol/L for males.Conclusions The TECHNICON IMMUNO 1 Testosterone assay demonstrates the analytical characteristics required for its utilization in the clinical laboratory.