An investigator was infected with Lassa virus while working with tissue cultures and mice infected with the virus. Prodromal symptoms of illness were malaise, shivering sensations, and severe pain in the lower portion of both thighs. A severe, acute, systemic, febrile illness ensued; its prominent features were myositis, myocarditis, and thrombocytopenia. Lassa virus was isolated from throat washings, blood serum, and urine. Complement-fixing antibody against Lassa virus was detected on the 52nd day after onset of illness. Administration of 500 ml of plasma from a person recovered from infection with Lassa virus, on the 10th day of illness, seemingly resulted in aborting the course of the disease.