  We present a non-local thermodynamic equilibrium abundance analysis of the
resonance doublet Li I at 6707.8 A for 55 Galactic F and G supergiants and
bright giants. The derived lithium abundances log \epsilon(Li) may be
considered in three groups, namely: (i) 10 Li-rich giants with log \epsilon(Li)
= 2.0-3.2 (all 10 are F-type or A9 stars); (ii) 13 G- to K0-type stars with Li
abundances in the narrow range log \epsilon(Li) = 1.1-1.8; (iii) all other
stars provide just upper limits to the Li abundance.
  The derived Li abundances are compared with theoretical predictions of 2-15
Msun stars. Our results are generally in good agreement with theory. In
particular, the absence of detectable lithium for the majority of programme
stars is explainable. The comparison suggests that the stars may be separated
by mass M into two groups, namely M < 6 Msun and M > 6 Msun. All Li-rich giants
and supergiants with log \epsilon(Li) > 2.0 have masses M < 6 Msun; 11 of 13
stars with log \epsilon(Li) = 1.1-1.8, specifically the stars with M < 6 Msun,
show good agreement with the post-first dredge-up surface abundance log
\epsilon(Li) = 1.4 predicted for the non-rotating 2-6 Msun stellar models. An
absence of Li-rich stars in the range M > 6 Msun agrees with the theoretical
prediction that F and G supergiants and giants with M > 6 Msun cannot show
detectable lithium.
  We note that present theory appears unable to account for the derived Li
abundances for some stars, namely for (i) a few relatively low-mass Li-rich
giants (M < 6 Msun), whose high Li abundances accompanied by rather high
rotational velocities or substantial nitrogen excess contradict theoretical
predictions; (ii) the relatively high-mass supergiants HR 461 and HR 8313 (M >
6 Msun) with the detected abundances log \epsilon = 1.3-1.5. It is possible
that the lithium in such stars was synthesized recently.
