Summary Twelve dairy herds that had participated in the Pennsylvania Dairy Herd Improvement Association (DHIA) program for at least 12 months, that had a 12-month mean DHIA somatic cell count > 700,000 cells/ml, and that had > 25% of lactating cows infected with Streptococcus agalactiae participated in a herd blitz treatment program. Initially, quarter milk samples for bacteriologic culturing were collected from all lactating cows. Subsequently, all cows identified as infected with Str agalactiae were treated, using a commercial penicillin-novobiocin intramammary infusion product. In addition, a herd mastitis management program of postmilking teat dipping and treatment of all cows at the start of the nonlactating period was instituted. Thirty days after the initial herd visit, samples from all lactating cows were again cultured, and cows infected at that time were treated. Twelve months after the initial herd visit, samples from all lactating cows were again cultured. Mean prevalence of infection with Str agalactiae decreased (P < 0.05) from 23.0% of quarters and 41.6% of cows initially to 3.4% of quarters and 9.3% of cows at 30 days and 1.6% of quarters and 4.2% of cows at 1 year. Mean herd DHIA somatic cell count decreased (P < 0.05) from 918,000 cells/ml initially to 439,000 cells/ml at 30 days and 268,000 cells/ml at 1 year. The mean net increase in 365-day DHIA rolling herd production for the first year after treatment was 512 kg of milk and 14 kg of butterfat. Benefit-to-cost ratios were estimated for 3 methods of identifying cows for treatment in a blitz treatment program. Analysis revealed similar economic gains for culturing of all lactating cows followed by treatment of those infected with Str agalactiae (benefit-to-cost ratio of 2.28:1), and for treatment of all lactating cows with a DHIA linear somatic cell count score ≥ 4, regardless of infection status (benefit-to-cost ratio of 2.18:1). Treatment of all lactating cows, regardless of somatic cell count or infection status, was least cost-effective (benefit-to-cost ratio, 1.26:1). It is suggested that blitz treatment, used with postmilking teat dipping and treatment of all cows at the start of the nonlactating period, is an economically justifiable program for herds with high prevalence of Str agalactiae mastitis, and owing to labor considerations, the use of DHIA linear somatic cell counts may be the best method to identify cows for treatment.