The effects of a toxic dose of Mycoplasma fermentans on levels of lysosomal enzymes in mice were examined. Washed cell suspensions (approximately 1010 colony-forming units) of a recent isolate of M. fermentans were injected intraperitoneally into 3to 4-week-old BALB mice, and levels of acid phosphatase and beta-glucuronidase were monitored in liver, spleen, thymus, and serum. Levels of acid phosphatase remained essentially normal, but levels of betaglucuronidase were markedly elevated in serum and to a lesser extent in liver and thymus. The peak response of serum beta-glucuronidase was noted at 8 h postinjection, with a level of 30 gg of phenolphthalein released per ml per h, representing a six-fold increase over control levels. Pretreatment with BCG did not potentiate the effect as it did with endotoxin. The implication of this increased lysosomal enzyme activity in "lethal toxicity" is that that the increase may be secondary to some other cytotoxic event, or that the affinity of mycoplasmas for biological membranes may be involved. The data suggest that the role of lysosomal enzymes in other models of mycoplasma-induced disease should be evaluated.