Antigenic extracts were prepared from culture filtrates of the principal agents known to cause actinomycetoma, namely Actinomadura madurae, Actinomadura pelletieri, Nocardia asteroides, Nocardia brasiliensis, Nocardia otitidis-caviarum, and Streptomyces somaliensis. These antigenic preparations were compared by immunodiffusion (ID), counterimmunoelectrophoresis (CIE), line immunoelectrophoresis (LIE) and rocket line immunoelectrophoresis (RLIE), with rabbit antisera prepared against each of the extracts. Cross-reactivity between antigenic extracts from the different actinomycetes, measured by determining the number of precipitin lines in homologous and heterologous systems, was common. Reactions were always stronger and more precipitin lines were present when antigenic extracts were tested against homologous antisera. Similarities between A. madurae and A. pelletieri antigens were of a low order and cross-reactivity did not exceed 33%. A. pelletieri resembled N. asteroides more closely than A. madurae, with 44% of detectable antigenic components cross-reacting. The three species of Nocardia had common antigenic epitopes, but the overall degree of similarity was of a low order (between 12 and 27% by LIE and RLIE). Antigenic extracts of S. somaliensis had few components in common with the other species tested and only one of the 34 lines present in the RLIE system for N. asteroides showed any reaction of identity with an antigenic component in the S. somaliensis extract. Single cross-reacting lines were also present in the CIE and LIE systems.