Using monoclonal antibodies (MoABs) against blood group determinants and related 
carbohydrate sequences, it is now possible to clarify their carcinoma-associated 
modulation at a molecular level. In the present study a panel of MoABs against 
different type 1 chain derived blood group antigens, comprising A, B, H type 1, 
Le(a), sialyl-Le(a) (CA 19-9), sialyl type 1 structure (CA 50), and Le(b) was 
used to investigate their immunoreactivity in 38 medullary carcinomas of the 
thyroid (MTC) and in normal thyroid tissue. The antigens were not expressed in 
normal follicular or C-cells but were expressed to a various extent in MTC. The 
studies revealed some characteristic anomalies in the frequency and patterns of 
tumor-associated antigen expression. The MoAB C 50 stained 32 of the 38 tumors, 
H type 1 (Le(d)) was demonstrated in 21 and the Le(b) antigen in 27. The Le(a)- 
and the A antigen were detected in 10 and 12 tumors and the B antigen in one. 
From the results some rules about the pathways for tumor-associated 
re-expression of these antigens can be deduced. Le(a) antigen expression was 
significantly correlated with the CA 50 and Le(b) antigens. The significant 
relation observed between A-, H1-, and Le(b) antigen formation in MTC suggests 
the existence of a carcinoma-associated fucosyltransferase committing the type 1 
precursor chain along the H1-antigen pathway, and by further glycosylation to an 
A-, B-, or a Le(b) antigen. Comparative studies of tumor-associated H type 1 and 
H type 2 antigen expression revealed that H type 2 antigen synthesis was 
significantly related to a blood type 0 in the host. On the other hand, H1 
antigen reactivity was independent of the AB0 blood type of the hosts and was 
also detected in H type 2 antigen-negative tumors. These findings support the 
proposal that even in tumor tissue, H antigen expression is still determined by 
the interaction of at least two different genes. Despite the occurrence of the 
precursor substance (CA 50) and the formation of the Le(a)- and Le(b) antigens, 
indicating the presence of a alpha 1,4-fucosyl-transferase (Lewis-enzyme), only 
two tumors showed the formation of CA 19-9. In conclusion, the investigations 
demonstrated the dominant re-expression of three type 1 chain-derived structures 
in MTC, namely H type 1, Le(b), and CA 50. These findings support the general 
concept demonstrated in other carcinomas, that fucosyl- and sialyltransferases 
are preferentially activated in MTC.(ABSTRACT TRUNCATED AT 400 WORDS)