Art Cashin said it appears some "real money must have come to work here" after stocks were able to sustain a sharp spike just after the start of trading.
Cashin, UBS' director of floor operations at the NYSE, told CNBC's Bob Pisani around midday that it's not entirely clear what fueled the buying, but one theory "has to do with the end of the month and people beginning to set up before the final day on Monday."
He adds, "We'll wait to see how long this staying power has, but it's impressive."
Cashin said it would be a good sign for the bulls if the S&P 500 stock index can stay above 1860, but "dipping back below 1850 would be a bit of a problem."
—By CNBC's Alex Crippen. Follow him on Twitter @alexcrippen.