The passive immunodiffusion reaction in a microfluidic system is investigated via the interaction between fluorescein human albumin conjugate and monoclonal antibodies. The topology of a microfluidic device suitable for this reaction was designed, and the device was fabricated using LIGA technology. A system for transitioning from the macro- to the microlevel is developed and produced. The dissociation constants and specificity of interaction in complexes formed as a result of the reaction between fluorescein human albumin conjugates and monoclonal antibodies are determined. The results from our study reveal that systems based on combinations of passive immunodiffusion reactions and microfluidic technologies are applicable for detecting antigens and antibodies in the analyzed fluids.