Sera from 71 patients with culturally proven nocardiosis were tested for precipitins against a pool of Nocardia asteroides and N. brasiliensis culture filtrates and against antigens from the supernatant of homogenized N. asteroides cells. A human nocardiosis case serum was used as a reference. Sera from 56 of the 71 cases were reactive with either the culture filtrate antigen, the homogenate antigen, or both antigens, resulting in an overall sensitivity of 79%. Sera from 35 of the patients (49%) were positive with the homogenate antigens, and 28 (39%) showed bands of identity with the reference serum. Sera from 50 nocardiosis cases (70%) were positive with the pooled culture filtrate antigens, and 29 (41%) produced bands of identity with the reference serum. Of 89 sera from patients with various systemic mycotic diseases, tuberculosis, or actinomycosis, 24 (27%) were positive with the nocardial homogenate antigens and 4 (4.5%) showed precipitin bands of identity. Thirty-five of the 89 sera (42%) were positive with the nocardial culture filtrate antigens, and 6 (6.7%) showed bands of identity. The majority of sera demonstrating false-positive reactions were from tuberculosis and actinomycosis cases. One of seven sera from well individuals produced a precipitin band with the culture filtrate antigen, but this was not a band of identity with reference serum. These antigens did not distinguish antibodies from patients with N. asteroides, N. brasiliensis, or N. cavia infections.