Objectives of this field investigation were to examine the etiology and mastitis infection dynamics in a 60 cow herd in danger of market loss, prioritize and implement prevention strategies, and evaluate the efficacy of recommended and extended therapy (S. aureus only) with pirlimycin hydrochloride (Pirsue, Pfizer Animal Health) for gram positive intramammary infections (IMI) . Initial percentages (October) of cows (quarters) uninfected or infected with Staph. aureus, Strep. dysgalactiae, alpha hemolytic strep., and Strep. uberis, were 42(65); 33(15%); 20(9); 3(1); and 2(0.4), respectively. Overall streptococci cure rates were 91% and 89% of cows and quarters, respectively and was higher than average published values (65-70%). Cure rates for Staph. aureus (extended pirlimycin therapy) were very low at 7% and 4% of cows and quarters, respectively. Overall results showed excellent therapy response for Strep. IMI but very low S. aureus cure rates. However, some therapy success coupled with critical and mandatory prevention strategies (including a strict milking order) resulted in a 50% decrease in SCC by January and limited new infections. Failure to continue these proper prevention strategies resulted in many new IMI and high herd SCC in February, and demonstrates the mandatory requirements for proper mastitis prevention strategies (including a milking order) in a comprehensive mastitis and milk quality program.