For the first time, a diagnostic test system for the determination of drugs, in particular, cannabinoids, in human urine was constructed on the basis of the latex agglutination reaction. For this purpose, polystyrene microspheres with a diameter of 0.6 μm and a narrow particle size distribution and carboxylic groups of stabilizer molecules on their surfaces were used. The studies revealed the optimal characteristics of particle suspension, which provide for a high sensitivity and specificity level of the test, in particular, the quantitative range of the antibody content for covalent binding with carboxylic groups on the surface of polystyrene microspheres. The operating titer of Δ-9-tetrahydrocannabinol (Δ-9-THC)-specific antibodies was 1/16-1/512 for a conjugated antigen taken at a concentration of 0.1 mg/mL. The sensitivity of the latex agglutination reaction when determining Δ-9-THC was 150 ng/mL.