"One hundred and twenty colostrum samples were obtained from 120 randomly selected populations of Holstein dairy cows just on day of calving in the south of Tehran. The competitive ELISA kit for serodiagnosis of anti Escherichia coli K99 (F5) attachment factor in colostrum (Biox diagnostics) were used. Out of 120 samples, 7 cases (5.83%) showed >80% inhibition in ELISA, which were considered as a high titre colostrum associated with E.coli K99, 28 samples had no any anti E.coli Ab, 71 samples showed degree of positivity between 1+ and 2+ and 14 samples were 3+. The present study showed that only 5.83% of colostrum samples had high titre of anti E.coli K99 Ab for passive transfer to protect the calves against diarrhea due to E.coli K99. Therefore, it is recommended that vaccination of pregnant cows is necessary for enhancing the titre of Ab in a high percent of them".