Prolonged milk residue in two cows after subcutaneous injections of penicillin at an extra-label dose.

After cesarian section was done on 2 Holstein cows, procaine penicillin G was injected SC at an extra-label-dose. The milk contained inhibitory residue for 21 days (case 1) and 10 days (case 2) after the final penicillin injection. The Bacillus stearothermophilus disk assay was used to confirm presence of an inhibitor. Analysis to specifically identify the inhibitor substance was not performed. Other sources of residues were examined and excluded in these cases. Procaine penicillin G was assumed to be the residue substance. Delayed drug clearance was attributed to depot formation following high-dose SC penicillin administration.