A man who worked as a legislative assistant to Rep. Shirely Chisholm (D-N.Y.) has filed a $550,000 lawsuit against the congresswoman and three of her female assistants accusing them of sex discrimination and "malicious" libel and slander.

The suit, brought by O'Dell Lewis, was filed in federal court here Friday.

Lewis charged that he was fired last February "on account of [my] sex," and subsequently was replaced by a woman who was not as qualified to hold the job. He said Chisholm initially told him he was being dismissed "because she had to cut back the number of employes on her staff."

About a month before his termination took effect, Lewis said, he hurt his back in a fall on the icy steps of a Capitol Hill office building, and was hospitalized. When he returned to close out his employment and to file a workman's compensation claim for his back injuries. Lewis said, he found the new employe, a woman, in his position.

Lewis said in the suit that Carolyn J. Smith, Chisholm's administrative assistant and also one of the four persons named in the suit, said she "heard" that he "was considering filing a complaint" against the congresswoman with the House Fair Employment Practice. Committee (FEPC), alleging sex discrimination.

He said Smith warned him: "If you file a complaint with FEPC, you will suffer. I am going to make sure that you never draw a nickel of [workman's compensation] money, and I'll have your --- taken off unemployment compensation"

Lewis filed a complaint with the FEPC on April 27, 1977. One month later, the complaint was dismissed.

Since then, Lewis said, he has not been able to receive workman's compensation or to hold a full-time job because Smith and two other staff aides named in the complaint have "maliciously given false and derogatory references" about him to the Department of Labor and to prospective employers.

Smith, Chsiholm's administrative aide, said yesterday that she and the other persons accused by Lewis would withold comment until they have had a chance to review the complaint. Also named in the suit are Muriel Morrissey, Chisholm's chief legislative assistant, and Colleen O'Connor, her mer press secretary.