We present FTIR spectrum of B polymorphic forms of famotidine (fam) that is a powerful histamine H2-receptor antagonist used in the treatment of peptic ulcer. Molecular mechanics and semi empirical AM1, PM3, MNDO and MINDO3 methods have been used to study the molecular geometry, and the harmonic vibrational spectra with the purpose to assist the experimental assignments of famotidine. The calculated geometric parameters have been compared to the corresponding X-ray structure of famotidine and it is found that AM I structure in agreement with the crystal data. We are also investigated the interaction of famotidine with ofloxacin which is a synthetic antimicrobial agent. The changes observed in the some bands (wavenumber, shape) of interacted compound indicated that there is a weak interaction between two molecules. PM3 calculations are also carried out to determine the possible molecular structure of the interacted compound. (c) 2004 Elsevier B.V. All rights reserved.