It is suggested that a recent determination of the mass and density of Pluto from observations of Neptune may be strongly influenced by small errors in the mass of Uranus or Saturn. The general agreement of the visually observed value for the diameter of Pluto (Kuiper 1950) with the upper limit derived from a near-occultation event (Halliday, Hardie, Franz, and Priser 1966) suggests that a value of 5500 km is most probably within 20 percent of the true value. A recent analysis of observations

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... Neptune (Duncombe, Klepczynski, and Seidelmann 1968) has shown that a minimum in the longitude residuals of Neptune is obtained when 0.18 earth mass is adopted for the perturbing mass of Pluto. A combination of this mass with the upper limit for the diameter yields a density 1.4 times the mean density of the earth or 7.7 gm cm 3 . Since the diameter is an upper limit the associated density value is apparently a lower limit. Although this value for Pluto's density is much more acceptable than the values of 40 (or larger) which were associated with the conventional value of 0.91 earth masses for Pluto (Wylie 1942), it is still much too large to fit theories of the formation of the * Contributions from the Dominion Observatory No. 267.