Brucella abortus is a bacterium which causes abortions and infertility in cattle 
and undulant fever in humans. It multiplies intracellularly, evading the 
mechanisms of cellular death. Nitric oxide (NO) is important in the regulation 
of the immune response. In the present work, we studied the ability of three B. 
abortus strains to survive intracellularly in two macrophage cell lines. The 
bacterial multiplication in both cell lines was determined at two different 
times in UFC/ ml units. Moreover the inoculated cells were also observed under 
light-field and fluorescence microscopy stained with Giemsa and acridine orange, 
respectively. The stain of both cellular lines showed similar results with 
respect to the UFC/ml determination. The presence of B. abortus was confirmed by 
electronic microscopy. In both macrophage cell lines inoculated with the rough 
strain RB51, the multiplication diminished and the level of NO was higher, 
compared with cells inoculated with smooth strains (S19 and 2308). These results 
suggest that the absence of O-chain of LPS probably affects the intracellular 
growth of B. abortus.