We reported that administration of anti-tumor necrosis factor alpha (anti-TNF-alpha) antibodies exacerbates the course of a Salmonella infection in both susceptible and resistant mice by preventing the suppression of bacterial growth in the reticuloendothelial system. In the present study, we evaluated the effect of in vivo neutralization of TNF-alpha on the histopathology of primary Salmonella infections. We show that in primary infections, the suppression of bacterial growth in the reticuloendothelial system coincides with granuloma formation in the spleen and liver. Administration of anti-TNF-alpha globulins on day -1 of salmonellosis affected neither the histological picture nor the course of the infection in the early stages of the disease (days 1 to 3), with splenic and hepatic lesions consisting mainly of polymorphonuclear leukocytes (PMNs); conversely, later in infection (days 3 to 7), the treatment inhibited the formation of granulomas. When the anti-TNF-alpha treatment was started well after the suppression of bacterial growth in the reticuloendothelial system and the formation of granulomatous lesions in the spleen and liver, a prompt relapse of the infection and regression of already established granulomas were seen. In anti-TNF-alpha-treated mice, salmonellae were found inside macrophages and PMNs and extracellularly in the necrotic tissue of the spleen, while in the liver the organisms were seen mainly in inflammatory mononuclear cells, resident Kupffer cells, and hepatocytes and occasionally in the extracellular compartment within necrotic lesions. The bacteria appeared most often in clusters, being morphologically intact when in the extracellular space or within hepatocytes, while undergoing various degrees of degeneration when inside phagocytes. The results suggest that TNF-alpha is required for granuloma formation in salmonellosis and that its neutralization does not completely abrogate the bactericidal activity of macrophages and PMNs. Salmonellae were observed to grow within both hepatocytes and phagocytes but were killed only in the latter.