To determine how long tumor cells can be stored without losing their immunocytochemical reactivity, five malignant pleural effusions in EDTA-coated tubes were analyzed after different storage times at either 4 degrees C or room temperature. Only minor differences were observed between the cells from the two storage conditions. Though there was a considerable decrease in the number of tumor cells attached to the slides from day 0 to 1, the number of tumor cells was still sufficient to allow their clear detection with the monoclonal antibody HEA-125 even on day 4 of storage in all cases. Therefore, for routine purposes, pleural fluids in EDTA-coated tubes can be stored for at least one day prior to immunocytochemical staining if the cells are gently handled during preparation. Pleural fluid is a richly nutritious medium not only for keeping cells alive but also for preserving their immunoreactivity.