Bill Gates would bet against bitcoin

Bill Gates is not on the bitcoin bandwagon.

The Microsoft founder said the cryptocurrency, which has a tendency to fluctuate wildly in value, is one of the "crazier, speculative things," during an interview with CNBC's "Squawk Box" Monday.

"As an asset class, you're not producing anything and so you shouldn't expect it to go up. It's kind of a pure 'greater fool theory' type of investment," he said. He added that if he could short it, he would.

There's been huge interest lately in bitcoin and other cryptocurrencies, electronic alternatives to government-issued money. But they still aren't widely used for buying goods and services. Most enthusiasts appear more interested in holding onto their digital coins -- hoping they'll rise in value -- than in using them for transactions. And high transaction fees, high volatility and minimal adoption by merchants have slowed bitcoin's adoption as a regular currency.

Gates also said someone gave him bitcoin for his birthday once and he ended up selling it a few years later.