A new class of promising antipneumocystis agents, cyclic lipopeptide pneumocandin analogs, has been shown to effectively prevent Pneumocystis carinii cyst development in murine models. These compounds are believed to inhibit the biosynthesis of beta-1,3-glucan, a major constituent of the cell walls of various pathogenic fungi. However, all evidence of the presence of this polymer in P. carinii cysts is based on indirect methods. To address this, highly specific rabbit polyclonal antiserum was raised against a laminariheptaose-human transferrin hapten conjugate. This antiserum was used to demonstrate the presence of beta-1,3-glucan in alkaline extracts of P. carinii-infected rat lung tissue and to quantitate the degree of infection in this tissue as laminarin equivalents. The antiserum was also used to localize beta-1,3-glucan in P. carinii-infected rat lung tissue at the transmission electron microscopic level by immunogold labeling. High concentrations of beta-1,3-glucan were present in the electron-lucent layer of the P. carinii cyst wall, but beta-1,3-glucan was absent from intracystic bodies and trophozoites. Ultrastructural evaluation of lung tissue from P. carinii-infected rats treated with the pneumocandin analog L-733,560 demonstrated that the few cysts which remained are deformed, lack the translucent layer of the cyst wall, and contain minimal amounts of beta-1,3-glucan.