Uruguay is situated in a marginal area for the development of Boophilus microplus (30 degrees 35 degrees South Lat.) with important areas of enzootic instability for Babesia bovis and B. bigemina. The livestock products represent 70% of our exports, for which reason it is fundamental to evaluate the losses in the production that these haemoparasites cause as basic information to take future decisions. In the period 1988-1990, several works were carried out by our laboratory to know the incidence of babesiosis in the reduction of liveweight gains. The results are shown and discussed in the work. Experiment I: the weight increase of the control group (x = 0.248 kg/day), was 23% higher than that of the infected group with Babesia spp (from Uruguay), but significant statistical differences were not found (P < 0.05). These animals were kept in boxes and the food was controlled for 76 days. Experiment II: the incidence of Babesia spp (same strain) was studied for 140 days on Hereford heifers (n = 14) on natural pastures. The control group obtained x = 25.29 kg of liveweight gain and it was 45% higher than that of the infected group, significant statistical difference were found (P < 0.05). Experiments with attenuated strains III: four studies were carried out inoculating B. bovis and B. bigemina in bovines about one year old, in different growth systems, searching for the limit of application. Significant statistical differences between those groups were not found during the experiment (about 180 days) (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)