We report the first detection of Ca II H and K absorption in the high-velocity cloud complex C, the largest HVC. Absorption is detected in the spectra of the quasar PG 1351+640 and of the Seyfert galaxy Mark 290, at velocities of -163 and -137km/s (see fig.2). The implied Ca+ abundances are about 2x10^-8^, or about 0.01 times the total solar Calcium abundance. This values lies near the middle of the range of values found for other HVCs. The measured abundance will allow firm conclusions about the distance of complex C from the presence or absence of its absoprtion in stellar spectra.