Coefficient of kinetic friction (m) between visceral and parietal pleura, sliding in vitro at physiological velocities and load, was found to increase 8 times after blotting mesothelial surface with filter paper; this increase was only partially reduced by wetting blotted mesothelium with Ringer solution (D'Angelo et al., Respir. Physiol. Neurobiol. 2004). Given that mesothelial surface is covered by a coat with sialomucin and hyaluronan, this result suggests that blotting removes part of coat macromolecules. In this research, we measured m of pleural mesothelium as previously described, with a more uniform blotting procedure, in order to determine whether addition of sialomucin (25 mg/ml) or hyaluronan (2.5 mg/ml) after mesothelium blotting lowers it more than that of Ringer. In sialomucin series (N=14) m was: initial Ringer 0.028 ± 0.003 (SE); blotting 0.143 ± 0.014; postblotting Ringer 0.083 ± 0.09; sialomucin 0.036 ±0.003; final Ringer 0.063 ± 0.007. Similar data were obtained with hyaluronan. These data show that addition of sialomucin or hyaluronan after mesothelium blotting lowers m more than that of Ringer (P < 0.01). This effect is such that after addition of these molecules m is no longer higher than initial one. Moreover, addition of Ringer after these molecules increases m (P < 0.01), in line with a dilution of added molecules by Ringer. Our results show that mesothelium blotting removes part of these molecules, and their relevance to mesothelium lubrication.