Aprospective, controlled-blind field trial was conducted toevaluate theefficacy andsafety ofparomomycin sulphate, given for10daysfrombirth, inpreventing natural cryptosporidiosis incalves. Intheuntreated control group,oocyst shedding anddiarrhoea followed thepattern that hadoccurred onthefarmbefore the trial, consistent with aperinatal infection, whereas inthetreated group,theprepatent period was significantly longer thaninthecontrol group(P<O-O1) andoocyst shedding anddiarrhoea started only after thedrug was withdrawn. However, theregimen didnotreduce theincidence ofdisease inthetreated group.