Cryptocurrencies are a speculative bet with little growth potential, famed billionaire investor Warren Buffett said on CNBC Monday.

Buffett said bitcoin (BTC), the world’s largest cryptocurrency, doesn’t produce anything except for more buyers looking to make money by selling to new buyers. He has voiced similar criticism of cryptocurrencies in the past.

The chairman of conglomerate Berkshire Hathaway, a self-proclaimed technophobe who tends to avoid investing in technology companies whose businesses he doesn’t understand, said he’s bullish on stocks and recently amassed a 5% stake in Apple. In the same interview, he lamented not having bought Google and Amazon early on.

Microsoft co-founder Bill Gates, speaking to CNBC in the same interview, also criticized cryptocurrencies as investments and said he’d sell short bitcoin if there was an easy way to do so. Bitcoin and other cryptos are “kind of a pure ‘greater fool theory’ type of investment,” he said.

The billionaire software developer and Berkshire board member spoke positively about the blockchain technology upon which cryptocurrencies are built, however. Read more here.