A myeloma cell line (KHM-4) from a patient with multiple myeloma and idiopathic hyperammonemia, and another myeloma cell line (RPMI8226) were seen to have activity to form ammonia from arginine. High activity of ornithine transcarbamylase (OTC), a hepatic urea cycle enzyme, was detected in these cell lines. OTC of these cells was much more heat-stable than the liver enzyme, and did not cross-react with an antibody against the liver enzyme. As the OTC activity was also detected in the culture medium of the myeloma cells and because the activity was markedly decreased by the antimycoplasma drug MC-210, the OTC activity was assumed to be associated with mycoplasma infection. Polymerase chain reaction, using degenerate oligonucleotide mixtures corresponding to the two highly conserved sequences of OTC, amplified a DNA sequence that apparently encodes a portion (about 67% in length) of mycoplasma OTC. The predicted amino acid sequence of the mycoplasma enzyme was 33-47% identical with those of the enzymes of bacteria, yeast and mammals.