Two novel monoclonal antibodies, KL-3 (IgM) and KL-6 (IgG1), which can detect soluble antigens in sera and effusions (molecular weights greater than 1,000 K) were produced against human pulmonary adenocarcinoma VMRC-LCR cells. KL-3 and KL-6 antibodies reacted with asialo- and sialo-carbohydrate antigenic determinants, respectively. Both carbohydrate epitopes appear, from competitive inhibition studies, to be different from Lex, Ley, sialyl Lea and sialyl Lexi which were recognized with FH2, AH6, NS19-9 and FH6 antibodies, respectively. Using an enzyme linked immunosorbent assay, elevated KL-6 antigen levels were frequently observed in the sera of patients with lung adenocarcinoma [52% (17/33)], pancreatic cancer [44% (4/9)] and breast cancer [40% (8/20)], but infrequently in the sera of patients with lung squamous cell carcinoma [18% (4/22)], lung small cell carcinoma [8% (1/13)], gastric cancer [0% (0/19)], colorectal cancer [0% (0/8)] and hepatocellular cancer [13% (1/8)]. The levels and positive rates of serum KL-6 antigen increased with the progression of clinical stage of lung adenocarcinoma. In pleural effusions, the prevalences of lung adenocarcinoma cases with elevated levels of KL-3 and KL-6 antigens were 76% (13/17) and 82% (14/17), respectively. These monoclonal antibodies can define novel soluble antigens in sera and effusions which could be useful in tumor diagnoses and for monitoring tumor progression.