The antiviral and virucidal effect of methanolic extract fraction of leaves of neem (Azadirachta indica A. Juss) (NCL-11) was studied regarding its activity and possible mechanism of action against Coxsackie B group of viruses. NCL-11 inhibited plaque formation in 6 antigenic types of Coxsackie virus B at a concentration of 1000 micrograms/ml at 96 hrs. 'in vitro'. Additionally virus inactivation, yield reduction and effect of time of addition assays suggested that NCL-11 was most effective against coxsackie virus B-4 as a virucidal agent besides interfering at an early event of its replicative cycle. The evidence suggested that presence of a battery of compounds besides flavonoids, triterpenoids and their glycosides in NCL-11 have antiviral action for coxsackie B group of viruses 'in vitro.' The minimal inhibitory concentrations were not toxic to Vero (African green monkey kidney), cells; subtoxic concentration was 8,000 micrograms/ml and cytotoxic concentration 10,000 micrograms/ml, which was confirmed by trypan blue dye exclusion test.