Izabela Miltko, a spokeswoman for the Service Employees International Union, says the janitors who were employed by Pritchard Industries Inc., were not allowed to return to work on Thursday, which she says violates federal laws.
Miltko says the union will file a complaint with the National Labor Relations Board.
Emails seeking comment from corporate officers listed on Pritchard's website were not immediately returned.
About 3,200 union-affiliated Houston janitors voted Tuesday to strike over what they say are threats and misconduct by janitorial contractors for some of Houston's biggest corporations.
The union says janitors in the city earn about $9,000 a year.