Compact colony-forming activity of strains of Staphylococcus aureus could quantitatively be determined by titrating for this activity in serum–soft agar containing serially diluted normal rabbit sera. The compact colony-forming activity values of 31 phage-propagating strains of S. aureus were compared. Results showed that significant variations were present in their activities; however, no phage-group specificity was shown among these strains. Further, with changes in the pH of the media, the compact colony-forming activity of a strain was remarkably decreased with increased acidity; however, this activity increased in alkaline media.