Marked plasmacytosis is an uncommon clinical finding associated with plasma cell dyscrasias and certain reactive states, particularly serum sickness. Moreover, serum sickness-like reactions are a well-recognized complication of therapy with streptokinase. In this report, the authors describe a patient who developed a transient, but striking, plasmacytosis and an unexplained fever following streptokinase treatment for a pulmonary embolus. An evaluation for multiple myeloma was completely negative except for the occurrence of serum monoclonal-like proteins which largely disappeared over an eight month period.