  Medium resolution J-band spectroscopy of individual red supergiant stars is a
promising tool to investigate the chemical composition of the young stellar
population in star forming galaxies. As a continuation of recent work on iron
and titanium, detailed non-LTE calculations are presented to investigate the
influence of NLTE on the formation of silicon lines in the J-band spectra of
red supergiants. Substantial effects are found resulting in significantly
stronger absorption lines of neutral silicon in non-LTE. As a consequence,
silicon abundances determined in non-LTE are significantly smaller than in LTE
with the non-LTE abundance corrections varying smoothly between -0.4 dex to
-0.1 dex for effective temperatures between 3400K to 4400K. The effects are
largest at low metallicity. The physical reasons behind the non-LTE effects and
the consequences for extragalactic J-band abundance studies are discussed.
