We derive the baryon mass function for a complete sample of low-redshift clusters and argue that it is an excellent proxy for the total mass function if the ratio f_b = M_b/M_(tot) in all clusters is close to its universal value, Ω_b/Ω_M. Under this assumption, the baryon mass function can be used to constrain the amplitude and slope of the density fluctuation power spectrum on cluster scales. This method does not use observational determinations of the total mass and thus bypasses major uncertainties in the traditional analyses based on the X-ray temperature function. However, it is sensitive to possible systematic variations of the baryon fraction as a function of cluster mass. Adapting a weak dependence f_b(M), suggested by observations and numerical simulations by Bialek et al., we derive σ_8 = 0.72 +- 0.04 and the shape parameter Ω_Mh = 0.13 +- 0.07, in good agreement with a number of independent methods. We discuss the sensitivity of these values to other cosmological parameters and to different assumptions about variations in f_b.
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