The levels of antibody to the excretory/secretory antigens of the infective larvae and adults of Toxocara vitulorum were measured by gel precipitation and ELISAs in the serum and colostrum of 12 buffalo cows naturally infected with T vitulorum and in the serum of their calves. The antibody levels were compared with the extent of T vitulorum infection as judged by faecal egg counts in the calves. The patterns of bands of the larval antigens and gel precipitating antibodies in cow serum taken one month before calving, in cow colostrum and in calf serum were very similar. Nine cows and their calves had gel precipitating antibodies but the remaining three cows and their calves did not. The ELISA detected anti-larval antibodies in the colostrum of all 12 cows and calves. With the exception of one calf there was a strong correlation (r = 0.902) between the antibody titre in cow colostrum and the titre of passively acquired antibody in calf serum. The titres of these passively acquired antibodies declined to their lowest levels in calves 12 to 25 days of age; the antibody concentrations then began to increase up to day 42 and remained stable for the remainder of the experiment (105 days). The titres of antibodies to the antigens of the adult worms, examined in four cows and their calves, were lower than the titres to the larval antigens; the calves absorbed this anti-adult antibody from the colostrum and the antibody levels reached a plateau between days 12 and 30 and remained stable for the rest of the experiment.(ABSTRACT TRUNCATED AT 250 WORDS)