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We review our method for finding substellar mass self-gravitating clumps (``proto-brown dwarfs'') in nearby molecular clouds, using mm-wave telescopes (Pound and Blitz 1993). The method represents a novel approach to looking for brown dwarfs and, for the nearest molecular clouds, has a greater mass sensitivity than traditional methods. We show how the method can distinguish between various functional forms of the low-mass ($< 0.2~{\rm M_{\sun}}$) stellar initial mass function, which is currently poorly constrained.
In both molecular line and continuum searches in the Ophiuchus B2 and Barnard 18 (in Taurus) star-forming regions, we find no clear-cut evidence of any proto-brown dwarfs. This negative result is surprising; even if the initial mass function were flat, we would expect to have found $\sim$ 10. There are a few objects near our detection limit ($0.01 {\rm M_{\sun}}$) which deserve further scrutiny. However, even if all these candidates turn out to be true proto-brown dwarfs, the IMF in Oph B2 in the lowest mass range cannot have a power-law index greater than $\alpha \sim 1$
Finally we discuss why the ambient pressure in molecular clouds may play a role in determining the IMF, and ultimately may create a low-mass cutoff.
