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An impediment to progress in the study of the course of growth, the effects of medium components, antibiotics, etc., of Mycoplasma has been the cumbersome methods of growth measurement currently in use. Heretofore, it required the plating of numerous samples during growth, at least in triplicate, after appropriate dilution, followed by a delay of 2 to 3 days before the colonies developed so that counts could be made. We applied the technique of light scattering to measure the growth of Mycoplasma laidlawii in liquid culture continuously in a manner analogous to the use of absorbancy for bacteria. Scattered light measurements precisely paralleled data obtained by the tedious method of plate counts and were available immediately during the development of the culture. The lower limit of sensitivity with the system described is 10(5)Mycoplasma per ml. The presence of serum in the medium lowers sensitivity somewhat. However, concentrations of serum up to 10% are easily tolerated. Higher serum content may require calibration curves. Thus the technique may be used with many pathogens, etc., that require serum to develop. One can easily and rapidly measure differences in growth rates as well as final cell yields during the course of growth, rather than 3 days later, after colonies have developed.