Andersen air samplers were used to determine the particle size distribution of Serratia marcescens aerosols created during several common laboratory procedures and simulated laboratory accidents. Over 1,600 viable particles per cubic foot of air sampled were aerosolized during blending operations. More than 98% of these particles were less than 5 ,u in size. In contrast, 80% of the viable particles aerosolized by handling lyophilized cultures were larger than 5 ,u. Harvesting infected eggs, sonic treatment, centrifugation, mixing cultures, and dropping infectious material produced aerosols composed primarily of particles in the 1.0to 7.5-,u size range. Considerable information has been compiled in recent years concerning the aerosol spread of microorganisms during routine bacteriological and virological procedures. Operations involving withdrawal of material from vaccine bottles (2, 6, 12), inoculation of mice and the inoculation and harvesting of eggs (6, 12), handling of lyophilized cultures (14, 15, 18), pipetting (2, 10, 16), use of the inoculating loop (2, 11), blending (2,