Mongrel dogs were anesthetized intraperitoneally with pentobarbitol. One cc of cerebrospinal fluid was drawn through a needle inserted into the cisterna magna and mixed with 1 cc (4-9 million viable BCG organisms) of freeze-dried bacillus Calmette-Guerin. One minute later this mixture was injected by the same needle into the cisterna magna. At 1 and 12 days postinjection, experimental animals were perfused with buffered aldehydes. Samples of the leptomeninges were post-fixed in OsO4 and routinely prepared for scanning and transmission electron microscopy. Leptomeningeal samples of untreated, control animals were similarly prepared. Scanning and transmission microscopy confirm that free cells resting on the subarachnoid linings and within the subpial connective tissue space of control animals possess the morphology of macrophages (Malloy and Low, '76). Viable BCG in the subarachnoid space produces a 3-fold increase in the free cell population of the leptomeninges in 24 hours and a 10-fold increase in 12 days. These cells tend to form associations varying from loose aggregates to tight clusters. Approximately 80% of these free cells express macrophage morphology, with abundant plasma-lemmal microappendages and cytoplasmic vacuoles. Transmission electron microscopy of the free cell population of BCG-stimulated animals reveals at least two other members of the leukocyte series on the leptomeningeal linings.