Our results showed that B. abortus strain RB51 was not iso-lated from any of the milk samples tested during nine weeks(63 days) post-vaccination. The use of selective media for direct plating of suspected samples could reduce the number of Brucellaspp. colonies, and consequently could result in reduction of sensi-tivity of the diagnosis (Marin et al., 1996). In order to overcome that and improve the sensitivity of the culture, reducing false-negative results, enrichment in tryptose broth supplemented with antimicrobials (Farrell’s supplement) was used before plating. This approach is reported to enhance the rate of isolation of B. abortus from infected animals by 50% (Minharro, 2009) and has an analyt-ical sensitivity of 200 B. abortus CFUs/mL of milk (data not shown).