Virginiamycin M factor transformation into an o-acetyl derivative has been observed in a particular Staphylococcus strain, naturally resistant to virginiamycin (Vr). It could not be detected in induced strains nor in sensitive ones. Turbidimetric studies with the naturally resistant strain showed that increasing concentration of the antibiotic had a progressive prolongation effect on the lag phase, without change in the growth rate. The prolonged lag phase could be reduced or eliminated by using an inoculum of cells previously "adapted" to M factor or virginiamycin. Preadapted cells had an increased acetylating power. Acridine dye treated cells showed a decreased o-acetyl derivative formation. This all suggested that virginiamycin resistance in the particular Vr strain was effectively connected with o-acetylation of the M factor.