An estimate of the second zonal coefficient of the spherical harmonic representation of the lunar gravitational field has been obtained from an analysis of particular orbital element variations of the Explorer 35 and Explorer 49 spacecraft. Data from these spacecraft were used because the orbital configurations resulted in variations of the longitude of periapse and which were, to first order, dependent only on the even zonal harmonics. In addition, the data time span for each satellite was extremely long; 2138 days for Explorer 35, and 230 days for Explorer 49. The value determined for the harmonic coefficients is C20= (-2.0272¿.0148) ¿10-4. Use of this value of C20, and values of &bgr;= (631.1¿4) ¿10-6 and &ggr;= (226.8¿1.0) ¿10-6 yields a value of the lunar moment of inertia of C/Ma2=0.392¿0.003.