Twenty-two antibodies with high affinity for AFP could be classified into groups according to five AFP binding regions, designated A-E, based on cross-inhibition studies and immunometric assay combinations. Antibodies in group A (ISOBM TD2 No. in parentheses): H31 (119), AFP-4-F45 (111), 140/5 (117), K51 (99), K52 (110), AFP-200014A (120) and F2 (118) and in group B: A4-4 (98) and AFP-4-F67 (93) were only inhibited by antibodies belonging to the same groups and could be used in immunometric assay combinations with all other antibodies. Groups C, D and E were inhibited by antibodies in adjacent antibody groups and did not form immunometric assay pairs with antibodies belonging to neighboring groups. Group C comprises: K28 (105), A34-B/B5 (101), AFP-4-F111 (95), AFP100025B (121) and K57 (116), group D: E7 (114) and D10 (92) and group E: H219 (115), K6B1 (103), A34-A/D12 (104), C2 (102), C9 (94) and C10 (97). For six of seven antibodies with low binding to labelled AFP, the specificity could not be determined. Two antibodies were not conclusively assigned to any binding region. Antibody 9A12 (109) may be classified into group E based on immunometric assay combinations, but could not be evaluated in cross-inhibition experiments due to low binding to labelled AFP. Antibody 19F12 (100) functioned as a tracer antibody in combination with all other antibodies, but did not bind AFP when used as solid phase antibody. This antibody could therefore represent a unique binding specificity.