A new electronic method has been developed that requires about 10 min to assay 252Cf sources between 1 μg and 50 mg. The assay is made by placing the 252Cf source in the center of a polyethylene moderator and measuring the resulting thermalized neutrons with fission counters in the moderator. All measurements are referenced to a 252Cf calibrated with a manganese sulfate bath by the National Bureau of Standards. Sources smaller than 1 μg and larger than 50 mg could be similarly assayed with appropriate modifications to the moderator. The new method is routinely compared with the manganese bath method to determine the precision and sensitivity of the measurements so that the new method can be used as a secondary standard. Measurements of 252Cf content agree with manganese sulfate bath determinations to within ±0.6%. The present design is tailored for a fission neutron spectrum but could be used with proper calibration in assaying between 106 and 1011 n/sec from sources having different spectra.