A new role of apolipophorin III (apoLp-III) as an immune activator has emerged recently. To gain insight into this novel function, the interaction of apoLp-III with lipopoly-saccharide (LPS) was investigated. ApoLp-III from Galleria mellonella was incubated with LPS from Escherichia coli O55:B5, and analyzed by non-denaturing polyacrylamide gel electrophoresis (PAGE). Protein staining showed that apoLp-III mobility was significantly reduced. In addition, silver and LPS fluorescent staining demonstrated that LPS mobility was increased upon incubation with apoLp-III. This result suggests association of apoLp-III with LPS. Sodium dodecyl sulfate (SDS) PAGE analysis showed decreased apoLp-III mobility upon LPS addition, indicative of LPS apoLp-III interaction in the presence of SDS. The unique tyrosine residue that resides in apoLp-III was used to provide additional evidence for LPS binding interaction. In the absence of LPS, apoLp-III tyrosine fluorescence was relatively low. However, LPS addition resulted in a progressive increase in the fluorescence intensity, indicating tertiary rearrangement in the environment of tyrosine 142 upon LPS interaction. Other well-characterized apoLp-IIIs were also examined for LPS binding. Manduca sexta , Bombyx mori and Locusta migratoria apoLp-III were all able to interact with LPS. The ability of apoLp-III to form complexes with LPS supports the proposed role of apoLp-III in innate immunity.