TEM showing MWCNTs engulfed by RAW264.7 macrophages in vitro. Cells were treated with MWCNTs (50 μg/ml) for 24 hr in serum-free defined medium and then fixed and embedded in agar and TEM performed as described in Methods. Panel A: Low magnification showing two cells with MWCNTs within cytoplasm (arrows). Panel B: Higher magnification of the inset frame from Panel A showing agglomerated MWCNTs and an individual MWCNT in cytoplasm (arrows). Panel C: Magnification of left hand inset frame from Panel B showing individual MWCNT in cytoplasm. Panel D: Magnification of right hand inset frame from Panel B showing aggregated MWCNTs (arrows). Panel E: Cytotoxicity of MWCNT compared to CBNP in RAW264.7 cells as measured by LDH assay.