The presence of specific IgE antibodies in the serum of patients with occupational asthma resulting from exposure to western red cedar (RCA) was studied by a radioallergosorbent test (RAST). The antigen matrix used in the RAST was either a conjugate of Sepharose particles with antigens in a crude cedar extract or with plicatic acid, the major haptenic component of cedar antigens. Of eighteen patients with clinical RCA and positive reaction to antigenic bronchoprovocation, eight were found to have abnormal RAST values. By appropriate absorption experiments, the serum RAST activity was shown to represent cedar antigen-specific IgE antibodies. No significant RAST activity was detected in the serum specimens from sixteen control subjects or from ten patients with negative bronchoprovocation-reaction to antigenic challenge. These results suggest that IgE antibody-mediated allergic reaction may be an important pathogenetic factor in RCA.