Digitalis intoxication is usually accidental in children. We report the case of a young infant with congenital heart disease in whom the coadministration of digoxin and josamycin led to a 50% increase in the digoxin concentration, generating sinoatrial block and cardiac failure. Clinical and electrocardiographic symptoms very quickly resolved following immunotherapy with antidigitalis Fab fragments. Digoxin concentrations must be carefully monitored in patients concomitantly receiving macrolides to ensure that the digoxin dose can be readjusted if necessary.