Four dry-cow treatment regimens were evaluated againstStaphylococcus aureusintramammary infections: 1) high persistency product at drying off and low persistency product 1 to 3 days prepartum; 2) high persistency product at drying oflF; 3) low persistency product 1 to 3 days prepartum; 4) untreated controls. Treatment 1 was no more effbzacious (64.4%) than Treatment 2 (61.3%). Both treatments were significantly different from spontaneous recovery observed in Treatment 4 (41.2%), but not significantly different from Treatment 3 (46.9%). Dry-cow therapy reduced newS. aureusintramammary infections during the dry period by half. Prepartum treatment eliminated more than 90% of newStreptococcus ubmisinfections.