A 13-fold increase (0.08 to 1.05%) in the isolation rate of Bacillus species from in-patient blood cultures led to the investigation of pseudobacteremia over a four-month period. Data on blood isolates (BACTEC NR 660, bottle types 6A and PEDS) of Bacillus species were compiled between July and October 1993. BACTEC bottles, as well as their plastic lids and rubber septa (representing 20% of stored bottles), and alcohol swabs, iodine preparation pads and butterfly collection systems were cultured. Air plates inside the BACTEC 660 instrument were cultured, as well as swabs of both the BACTEC needles and the interior of the BACTEC handling area. To investigate carry-over contamination, sterile BACTEC bottles were tested immediately following bottles containing Bacillus species. Growth of Bacillus species was obtained from 16% of the plastic lids and 1.3% of the rubber septa. No growth was obtained from other cultures. The outbreak coincide with construction on a driveway of the hospital over the area where BACTEC bottles were stored. Upon completion of construction in November 1993, the isolation of Bacillus species returned to baseline. No pseudobacteremia isolates occurred in areas where the trained intravenous team was assigned. In conclusion, hospital construction leading to airborne spread of Bacillus species may cause Bacillus species pseudobacteremia, and failure to disinfect blood culture bottles adequately may predispose to such an outcome.