In the post-transplant period, antimicrobial agents are often coadministered with cyclosporine (CsA) to treat the infections occurring in the immunosuppressed patients. These agents produce drug interactions with cyclosporine and can increase or reduce the blood concentration of the immunosuppressant. We report two cases of drug interaction between cyclosporine and two antimicrobial agents, josamycin and rifampicin, coadministered in a kidney-transplanted and a liver-transplanted patient, respectively. Josamycin increased the CsA blood levels by inhibiting the CsA metabolism through the hepatic cytochrome P450 enzymes. Conversely, rifampicin decreased the CsA blood levels by stimulating the same enzymatic system. When using these agents it is necessary to adjust the CsA doses to avoid risks of CsA toxicity or allograft rejection.