A total of 73 breeding-age and primigravid Jersey heifers in 4 herds was randomly allotted to treatment and control groups according to expected calving date. Thirty-five heifers were injected intramammarily with a nonlactating cow product containing penicillin/streptomycin. Thirty-eight heifers served as untreated controls. Of the 35 treated heifers, 34 (97.1%) were infected at time of treatment. In the untreated control group, all 38 heifers (100%) were infected at treatment time. At parturition, prevalence of intramammary infection in treated heifers decreased to 40%, whereas in the control group, prevalence remained about the same (97.4% of heifers). Prevalence of Staphylococcus aureus mastitis in treated heifers was reduced from 17.1% to 2.9% after treatment. In the control group, prevalence of S aureus mastitis decreased from 26.3% to 15.8%. Heifers treated during the second trimester of pregnancy had the greatest reduction in prevalence of mastitis and in somatic cell count at parturition, compared with controls. Findings indicated that intramammary treatment during pregnancy in primigravid heifers was effective in reducing prevalence of mastitis and somatic cell counts at parturition.