Meliacine (MA), a peptide isolated from leaves of the high plant Melia azedarach L inhibited the multiplication of foot and mouth disease virus (FMDV) in BHK-21 cells. In this report, we establish that the MA-inhibitable process takes place within the first hour of the viral reproductive cycle. MA had no virucidal effect and did not affect adsorption and penetration of the virus in cells. In experiments with neutral red-labeled virus, it was found that MA significantly suppressed the development of photoresistance of the virus in infected cells. In untreated cultures nearly all virus which adsorbed to cells was uncoated within 1 h at 37 degrees C, whereas in treated cultures, even after 3 h only 3% of the virus was uncoated. Labeling of BHK-21 cells with acridine orange showed that MA affects the pH of intracellular acidic vesicles. Therefore, it is concluded that MA prevents the process of uncoating of FMDV in BHK-21 cells by inhibiting vacuolar acidification.