OBJECTIVE: The present study aimed at elucidating the influence of polymorphic stability of lipid excipients on the physicochemical characters of different solid lipid microparticles (SLM), with the focus on the alteration of protein distribution in SLM. METHODS: Labeled lysozyme was incorporated into SLM prepared with different excipients, i.e. trimyristin (TG14), glyceryl distearate (GDS), and glyceryl monostearate (GMS), by water-oil-water (w/o/w) or solid-oil-water (s/o/w) method. The distribution of lysozyme in SLM and the release of the protein from SLM were evaluated by confocal laser scanning microscopy. The storage stability of SLM was characterized by HPLC, differential scanning calorimetry, X-ray powder diffraction, and scanning electron microscopy. RESULTS: Lysozyme was displayed as small scattered domains inside GDS and GMS SLM, whereas it was incorporated in the core of TG14 SLM formulated by the w/o/w method or evenly distributed in TG14 SLM prepared by the s/o/w method. Stability study at 37 °C revealed that only TG14 SLM made by the w/o/w method was able to maintain the lysozyme amount both on the particle surface and released from the SLM. Elevated storage temperature induced polymorphic transition of lipids in GDS and GMS SLM, which was, however, not remarkable for the TG14 SLM. CONCLUSIONS: Lipid excipients and particle preparation methods were found to differently affect the lysozyme distribution in SLM, owning to varied storage stabilities of the lipids. The present study provides updated knowledge for rational development of lipid-based formulations for oral delivery of peptide or protein drugs.