A helical mycoplasma, Spiroplasma mirum strain SMCA, produced malignant transformation in mouse NIH 3T3 cells and monkey kidney CV-1 cells. The transformed cells exhibited morphological changes consistent with the transformed phenotype, grew in soft agar and produced tumors in athymic and BALB/c mice. Transmission electron microscopy revealed structures morphologically similar to mycoplasmas present in the cytoplasm of transformed but not untransformed 3T3 cells. The time of inoculation of S. mirum SMCA to 3T3 cells and the passage level of 3T3 cells affected transformation.