SUMMARYNuclease activity was observed in several saprophytic and parasitic Mycoplasma organisms ; the nucleases of Mycoplasma laidlawii were studied in detail. Nuclease activity of this organism was highest a t the early logarithmic phase of growth, and was found mainly in the soluble fraction of the organisms. Anion-exchange chromatography of the proteins of the soluble fraction separated ribonuclease from deoxyribonuclease activity. Each enzyme had an alkaline pH optimum, required magnesium ions for activity, and degraded native and heat-denatured nucleic acids. M . laidlazvii RNase degraded RNA-core. RNA inhibited the degradation of DNA by M . laidlawii deoxyribonuclease. The implications of these findings with respect to the effects of RNA and DNA on growth of M . laidlazvii are discussed.