The preventive and therapeutic efficacy of halofuginone-lactate (HFL) against Cryptosporidium parvum was evaluated in a study using 260 spontaneously infected calves. HFL was administered orally for 7 days to 1-day-old and 8-day-old calves, respectively.In both treated groups the drug significantly, and in almost the same manner, decreased the intensity of diarrhoea and faecal oocyst count when compared to corresponding placebo groups. The only difference between treated groups was the time of onset ofsymptoms of the infection. The results confirm the anticryptosporidial activity of HFL in calves, but show that the outcome of infection following preventive treatment is comparable to that observed in calves treated after the onset of symptoms.