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 overcomethat and improve the sensitivity of the culture, reducing false-negative results, enrichment in tryptose broth supplemented withantimicrobials (Farrell’s supplement) was used before plating. Thisapproach is reported to enhance the rate of isolation of B. abortusfrom infected animals by 50% (Minharro, 2009) and has an analyt-ical sensitivity of 200 B. abortus CFUs/mL of milk (data not shown).