Microbial growth in a Todd-Hewitt broth has been followed to determine the in-vitro post-antibiotic effects of penicillin in a Lancefield group A streptococcal strain. Bacteria were exposed for 2 h at 37 degrees C to 1 x MIC of penicillin. Following antibiotic removal, inactivation with penicillinase and regrowth in a drug-free broth, the duration of the effect was found to be 2.8 h. By studying the affinity of streptococci for xylene in the post-antibiotic phase we observed that the penicillin treatment had no effect on the cell surface hydrophobicity. The ability of the same streptococci to adhere to human buccal epithelial cells was greatly reduced. Streptococci exposed to penicillin produced much more deoxyribonuclease and hyaluronidase than control bacteria.