A series of 152 quarters affected with subclinical staphylococcal and streptococcal mastitis was treated at random by intramammary infusion with one of 4 formulations containing inert solvent, specially prepared immunoglobulin (Ig) and antibiotics (penicillin G and streptomycin) in various combinations. The effect of treatment was measured by assessing the conversion of mastitic quarters to a normal or mastitis-negative state over a post-treatment period of 28 days. Considering the solvent as a placebo, solvent plus Ig had no significantly better effect than solvent only; solvent plus antibiotics gave results that were highly significant after 14 days even though about a third of the strains of S. aureus were resistant in vitro to one or both of the antibiotics; solvent plus antibiotics and Ig gave results that were highly significant during the whole 4 weeks after treatment. The Ig solution appeared to be considerably more effective against staphylococcal than streptococcal mastitis, but the small numbers precluded statistical analyses. It was concluded that use of the particular batch of Ig alone had no value in the treatment of the common forms of subclinical bacterial mastitis but that in combination with antibiotics the Ig had a somewhat better effect than was obtained with antibiotics alone.