The dimorphic transition of Candida albicans from the yeast (Y-Candida) to the hyphal (H-Candida) form is a complex event whose relevance in fungal pathogenicity is still poorly understood. Using a cloned macrophage (M phi) cell line (ANA-1), we have previously shown that a M phi can discriminate between the two fungal forms, eliciting different secretory responses. In the present study, we investigated the susceptibility of Y-Candida and H-Candida to M phi proteolytic activity. In particular, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot (immunoblot) techniques were employed to analyze the patterns of lyticase proteinaceous extracts from cell walls of Y-Candida and H-Candida which had been unexposed or exposed to ANA-1 M phis for 3 h. Silver staining allowed detection of a complex protein pattern in both forms of C. albicans, qualitatively and quantitatively differing from each other, mainly at molecular masses below 106 kDa. Western blot staining with anti-C. albicans mannan antibodies and convalescent-phase sera of mice previously infected systemically or intracerebrally with C. albicans showed that, after contact with M phis, Y-Candida but not H-Candida proteinaceous cell wall components are profoundly modified, with substantial reduction and/or disappearance of many bands. Our experimental approach provides initial insights into the differential susceptibility of Y-Candida and H-Candida to the proteolytic activity of M phis.