The expression of the lacto-N-fucopentaose III (LNF III) epitope by tumor cells of the gastrointestinal tract and ovary has been observed in tissue sections with the use of the murine monoclonal antibody GA 29–1. The presence of the LNF III epitope in the circulation of patients with colorectal cancer has also been reported. In this preliminary study, we describe the presence of LNF III activity in the serum of patients with adenocarcinoma of the ovary. Twelve of 18 (66%) patients with stage I-IV disease demonstrated high reactivity to the GA 29–1 monoclonal antibody. This serum reactivity was independent of disease stage and histologic cell type. In contrast, only 1 of 6 control patients demonstrated a false-positive level of reactivity to GA 29–1. These preliminary results suggest that LNF III warrants further study of its potential application as a serum tumor marker test in patients with adenocarcinoma of the ovary.