The Collaborative International Dictionary of English v.0.48:

Leuc- \Leuc-\ (l[=u]k-) pref.
   Same as Leuco-.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Leuco- \Leu"co-\ (l[=u]"k[-o]-), Leuc- \Leuc-\ (l[=u]k-). pref.
   [Gr. leyko`s white.]
   A combining form signifying white, colorless; specif.
   (Chem.), denoting an extensive series of colorless organic
   compounds, obtained by reduction from certain other colored
   compounds; as, leucaniline, leucaurin, etc.
   [1913 Webster]