quantitation of viable mycoplasmas is described. Serial dilutions were made in sterile microtiter plates with standard microtiter equipment. The results were multiplied by a factor of 2.38 to obtain colony-forming units comparable to those obtained with the laborious glass pipette-tube dilution method. Because of the difficulty involved in obtaining viable counts of mycoplasmas, we suggest the use of a simple method for determining colonyforming units (CFU). There are many reports in the literature describing quantitation of mycoplasma growth by studying viable counts, opacity, hemagglutination, dry weight, pH change, protein production, glucose consumption, and cholesterol consumption (1, 2, 5, 7). Although these methods appear to be adequate for many studies, they are not suitable when a simple and exact quantitation of mycoplasma growth is required. We have found that quantitation of mycoplasma growth can be done with sterile microtiter plates and standard microtiter equipment. This dilution method is not only inexpensive, rapid, and simple, but also reproducible, as compared to the laborious glass pipette-tube dilution methods frequently used. Mycoplasma pneumoniae was obtained from the American Type Culture Collection (strain 15293), and Mycoplasma salivarium was obtained from E. D. Brastins, University of Pittsburgh. These organisms were maintained in mycoplasma broth (PPLO broth [Difco Laboratories, Detroit, Mich.] supplemented with 20% horse serum [GIBCO Laboratories, Grand Is

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The accuracy of the 0.001-ml calibrated platinum loop was tested by two methods: visible absorption spectroscopy and weight determinations. By both methods it was demonstrated that the volume of the loop delivery is related to the angle at which the loop is withdrawn from the solution being sampled and the diameter of the container, provided that the volume in the container is adequate to cover the loop. Vertical sampling from small-diameter containers (less than or equal to 7-mm inside diameter) delivered approximately 50% of 0.001 ml, and sampling at a 45 degree angle from larger containers (greater than or equal to 22-mm inside diameter) gave a delivery volume of greater than 150% of 0.001 ml, resulting in a threefold difference in the amount delivered and an error rate of +/- 50%.

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Enamel slabs and thin sections with artificially formed caries lesions were placed in the removable dentures of 15 subjects who brushed them three times daily for 2 weeks with a 1.3% pyrophosphate/1,100 ppm F/1.5% Gantrez dentifrice, a 1,100-ppm F dentifrice without pyrophosphate or Gantrez, or a placebo dentifrice in a double-blind, crossover study. Analysis of the thin sections by quantitative microradiography showed that the pyrophosphate/F dentifrice remineralized the lesions 15.9% (p < 0.05) and the fluoride dentifrice 11.2%, whereas the placebo produced 4.4% demineralization. Fluoride uptake by the enamel slabs was similar for both fluoride dentifrices and significantly greater from the placebo dentifrice (p < 0.05). The findings suggest that the anticaries mechanisms of fluoride include not only the prevention of demineralization, but also the promotion of remineralization.