Freelite κ and λ serum free light chain (sFLC) assays use polyclonal antisera directed against the “hidden” epitopes of FLC molecules that are located at the interface between the light and heavy chains of intact immunoglobulins (Figure 5.1). These epitopes are only accessible when light chain molecules are not associated with the immunoglobulin heavy chain. κ and λ FLCs are measured in pairs to produce κ/λ sFLC ratios, or calculate the difference between the involved and uninvolved sFLC concentrations (Section 7.2.2). Polyclonal antibodies raised in sheep provide the most attractive method of recognising the highly polymorphic FLC molecules (Chapter 3). Latex enhancement increases the sensitivity of Freelite assays, to a few mg/L, and the assays are performed by turbidimetry or nephelometry on a number of automated laboratory instruments (Chapter 37).