  The equivalent widths of C II $\lambda$ 4267 \AA line were measured for the
mass-gaining primary stars of the 18 Algol-type binary systems. The comparison
of the EWs of the gainers with those of the single standard stars having the
same effective temperature and luminosity class clearly indicates that they are
systematically smaller than those of the standard stars. The primary components
of the classical Algols, located in the main-sequence band of the HR diagram,
appear to be C poor stars. We estimate $ [N_{C} /N_{tot}] $ relative to the Sun
as -1.91 for GT Cep, -1.88 for AU Mon and -1.41 for TU Mon, indicating poorer C
abundance. An average differential carbon abundance has been estimated to be
-0.82 dex relative to the Sun and -0.54 dex relative to the main-sequence
standard stars. This result is taken to be an indication of the transferring
material from the evolved less-massive secondary components to the gainers such
that the CNO cycle processed material changed the original abundance of the
gainers. There appear to be relationships between the EWs of C II $\lambda$
4267 \AA line and the rates orbital period increase and mass transfer in some
Algols. As the mass transfer rate increases the EW of the C II line decreases,
which indicates that accreted material has not been completely mixed yet in the
surface layers of the gainers. This result supports the idea of mixing as an
efficient process to remove the abundance anomaly built up by accretion.
Chemical evolution of the classical Algol-type systems may lead to constrains
on the initial masses of the less massive, evolved, mass-losing stars.
