Art Cashin said the Nasdaq is "having a good run" but has a technical barrier ahead of it.
Cashin, UBS' director of floor operations at the NYSE, told CNBC's Bob Pisani around midday the Nasdaq "should run into some resistance at the 4150-4165 area."
With two hours remaining in the session, the index was trading around 4118. It went as low as 4014 on Monday.
Cashin also advised investors to be "a little cautious, be nimble" with the government reporting April jobs numbers Friday morning. He's hearing non-farm payrolls growth is expected to be between 180,000 and 220,000.
—By CNBC's Alex Crippen. Follow him on Twitter