The effects of purified human airway lysozyme and hen egg-white lysozyme on growth rate and viability of growing type I Streptococcus pneumoniae were studied. Exposure of bacteria to human and hen lysozyme at the same final concentration (100 micrograms/ml) for 1.5-4.5 h resulted in a marked reduction of the number of colony-forming units per ml compared to control cultures. After a 1.5-h exposure to human or hen lysozyme, the remaining percentage of colony forming units per ml was 54% and 69%, respectively. The onset of growth only appeared after a 3.5-h exposure period for human lysozyme whereas it began at 2.5 h for hen lysozyme. After 3.5 h and 4.5 h of exposure, the number of colony-forming units was significantly lower (p less than 0.05) in human lysozyme-treated bacteria cultures compared to control cultures. Parallel electron microscopic observations of Streptococcus pneumoniae cultures confirmed that the density of pneumococci was less in the presence of either human lysozyme or hen lysozyme in comparison to control cultures, and showed the presence of numerous long, ribbon-like material and cytoplasmic condensations liberated in the culture medium.