The present investigation examined the morphological characteristics of epiplexus macrophages following a single intracisternal injection of the antigen, bacillus Calmette-Guerin (BCG). Three days following injection of BCG (0.5 - 4.0 X 10(8) viable microorganisms), mongrel dogs were perfused with buffered aldehydes. The choroid plexus of the lateral and third ventricles was removed and routinely prepared for scanning or transmission electron microscopy. Choroid plexuses from normal animals (no BCG injection) were similarly prepared. Macrophages of normal animals possessed smooth cell surfaces with usually one to three cytoplasmic processes. Following BCG injection, a 10-fold increase in the epiplex macrophage population was observed. Furthermore, the majority of these cells presented an abundance of cell surface microappendages; including blebs, ruffles and microvilli. Cell processes and microvilli frequently mediated contacts between widely separated macrophages. These associations may play a role in the initiation and/or maintenance of the cellular immune response to BCG.