We present the detection of molecular absorption lines in the optical spectrum of the post-AGB star HD 56126. The C_2_ Phillips A^1^{PI}_u_-X^1^{SIGMA}^+^_g_(1,0), (2,0), and (3,0); Swan d^3^{PI}_g_-a^3^{PI}_u_(0,0) and (1,0); and CN Red system A^2^{PI}-X^2^{SIGMA}^+^ (1,0), (2,0), (3,0), and (4,0) bands have been identified. From the identification of the molecular bands we find an expansion velocity of 8.5+/-0.6km/s independent of excitation condition or molecular specie. On the basis of the expansion velocity, rotational temperatures, and molecular column densities we argue that the line-forming region is the AGB remnant. This is in agreement with the expansion velocity derived from the CO lines. We find column densities of logN_C_2__=15.3+/-0.3cm^-2^ and logN_CN_=15.5+/-0.3cm^-2^, and rotational temperatures of T_rot_=242+/-20K and T_rot_=24+/-5K respectively for C_2_ and CN. By studying molecular line absorption in optical spectra of post-AGB stars we have found a new tracer of the AGB remnant. From comparison with the results of CO and IR observations it is possible to obtain information on non-spherical behavior of the AGB remnant. Using different molecules with different excitation conditions it should be possible to study the AGB remnant as a function of the distance to the star, and thus as a function of the evolutionary status of the star on the AGB.