A new primary radioactivity standardization of 237Np was performed. Neptunium-237 is produced as a long-lived waste product in nuclear reactors. Hence, accurate standardization is necessary for environmental monitoring of nuclear waste. The standardization of 237Np by several primary methods was investigated. This was performed to support a new 237Np transfer standard that was developed and which will be disseminated by the National Institute of Standards and Technology (NIST) as Standard Reference Material SRM 4341a. A EUROMET comparison hosted by NPL in 1998 identified 237Np-233Pa equilibrium/stability issues. As a result of this study our master solution was diluted in steps, and stability tested at every step. Data suggested that the equilibrium was disturbed when aliquots were removed from the ampoule; hence sufficient time elapsed before standardization measurements were performed. The certified massic activity of SRM 4341a as obtained from the 4ηαβ liquid scintillation based standardization could be directly compared to the results obtained from the weighted mean of 9 primary standardizations by 5 laboratories and performed in 1998-99 as part of the EUROMET 237Np measurement comparison (-0.07 %). NIST confirmatory standardizations of the 237Np massic activity for SRM 4341a were performed by live-timed anticoincidence (LTAC) 4ηβ(LS) - γ(NaI) measurements and by high-resolution HPGe gamma-ray spectrometry (γ-spec) with a comparison difference of -0.13 % and 3.8 %, respectively. The uncertainty in the (γ-spec measurement was 6.5 % (k = 1). SRM 4341a was in agreement with the previous issue of 237Np (SRM 4341), first disseminated in 1993, to within 0.03 %. The 237Np SRM solution standards are contained in 5 mL flame-sealed borosilicate glass ampoules, and consist of (5.3196 ± 0.0003) g of a carrier-free nominal 2 mol·L-1 nitric acid solution, having a density of (1.067 ± 0.002) g·mL-1 at 16.3 °C. The combined standard uncertainty (κ = 2) on the standardization is 0.92 %.