The equivalent widths (EWs) of the C iiλ4267 Å line were measured for the mass-gaining primary stars of 18 Algol-type binary systems. The EWs of the gainers were compared with the EWs of single standard stars that have the same effective temperature and luminosity class. This comparison clearly indicates that the EWs of the gainers are systematically smaller than those of the standard stars. The primary components of the classical Algols, located in the main-sequence band of the Hertzsprung–Russell diagram, appear to be carbon-poor stars. We estimate [NC/Ntot] relative to the Sun as −1.91 for GT Cep, −1.88 for AU Mon and −1.41 for TU Mon, indicating poorer carbon 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 material transferring 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 the C iiλ4267 Å line and the rates of orbital period increase and mass transfer in some Algols. As the mass transfer rate increases, the EW of the C ii line decreases. This indicates that accreted material has not yet been completely mixed 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. The chemical evolution of the classical Algol-type systems could lead to constraints on the initial masses of the less massive, evolved, mass-losing stars.