A man with acquired immunodeficiency syndrome and multiple opportunistic infections developed central nervous system signs, and results of computed tomography suggested brain abscess. Histologic study and culture of material obtained from the brain biopsy specimen revealed Aspergillus. The clot from a peripheral venous blood sample obtained at the time of biopsy was processed and cultured on a monolayer of L cells. Five days later, Toxoplasma tachyzoites were identified in the cell culture. We bring this case to the attention of the medical community to point out that inoculation of tissue cell cultures as for viral isolation may aid in the diagnosis of Toxoplasma infection in these and other immunocompromised patients.