The role of serum in the production of tumor necrosis factor-alpha (TNF-alpha) by human monocytes stimulated with yeast-form Candida albicans was studied. Pre-exposure of C. albicans to human pooled serum enhanced both TNF-alpha mRNA and cytokine secretion compared with C. albicans preincubated with medium only. Serum factors involved were >30 kDa, were efficiently inhibited by D-mannose, and recognized both Ca++-dependent and -independent pathways. Preincubation of yeasts with rabbit mannose-binding protein (MBP) resulted in dose-related enhancement of TNF-alpha secretion, through a Ca++-dependent pathway inhibited by D-mannose. TNF-alpha levels were similarly induced in C. albicans preincubated with vitronectin and with serum. Ca++ depletion did not affect cytokine release, while D-mannose supplementation displayed inhibition. The latter effect was abolished after Ca++ depletion. These data call for an involvement of both MBP and vitronectin in the serum-mediated enhancement of TNF-alpha release upon stimulation of monocytes with yeast forms of C. albicans.