"Short-Pen" Pen G, 100 ml Penicillin G Procaine is a potent antibacterial agent which is effective against a variety of pathogenic organisms, chiefly in the gram-positive category. PEN-G is recommended for the treatment of bacterial pneumonia (shipping fever) caused by Pasteurella multocida in cattle and sheep, erysipelas caused by Erysipelothrix insidiosa in swine and strangles caused by Streptococcus equi in horses.