An analysis of cell cycle-dependent expression of tumor-associated antigen was performed on a human neurosarcoma cell line (T2 cells). The expression of sarcoma-associated tumor antigen on T2 cells was detected using test sera obtained from sarcoma patients; control sera were from patients with nonsarcoma neoplasias and from normal donors. Results indicate a progressive increase in the antigenic expression beginning in late mitosis and early G1 with maximum expression in mid-G1. Antigenic expression declines to minimum levels in S and G2-phase. Mechanisms responsible for this cycle-dependent fluctuation are presently unknown.