Antibiotic-resistant Staphylococcus aureus strains of phage type 88, lysogenic for phage 188, when grown in mixed culture with a nonlysogenic novobiocin-resistant strain, acquired novobiocin resistance at a high rate from the nonlysogenic strain. With most strains of phage type 88, there was no detectable transfer of resistance from lysogenic to nonlysogenic cells. Lysogenization with phage 188 of phage-sensitive strains conferred on the lysogenized cells the ability to acquire chromosome and plasmid resistance markers. The acquisition of novobiocin resistance in liquid cultures depended on the aeration of the culture, cell density, and the presence of Ca2+. Pronase, and to a lesser degree other proteinases, increased the rate of acquisition of chromosome- and plasmid-determined resistance markers by cells lysogenic for phage 188.