CAMPATH-1H (CP-1H) is a humanized monoclonal antibody directed against the CD52 antigen with promising therapeutic effects in patients with small cell lymphocytic non-Hodgkin's lymphomas (NHL) of B- and T-cell type. We report about the response and toxicity of CP-1H in 18 patients with B-cell NHL who were treated in four clinical centers in Germany. Sixteen patients suffered from a low-grade and two from a high-grade NHL. All patients had received chemotherapy before and had either relapsed or were refractory to conventional therapy. Two patients received CP-1H in a dose-range finding trial once weekly and 16 patients as a fixed dose of 30 mg three times weekly. Of 18 patients, 8 (44%) achieved a clinical response, 2 (11%) had stable disease, and 5 (28%) had progressive disease. Four patients could not be evaluated for response because of death (two patients) and serious adverse events (two patients). All patients with response to CP-1H had a low-grade NHL. Nonhematological toxicity was severe in two patients who suffered from WHO grade III/IV bronchospasm. Common acute adverse events (WHO grade I-III) included fever, chills, rigor, urticaria, nausea, and vomiting. Eleven patients suffered from bacterial or viral infections; some had recurrent infections. A total of 12 different infections were reported. The most frequent infections were caused by herpesvirus (seven patients). Hematological toxicity included thrombocytopenia in four and lymphocytopenia in seven patients. Although the antibody is humanized, the nonhematological toxicity was substantial and probably due to a cytokine release syndrome. Prophylactic treatment of the side effects is strongly recommended for patients treated either with CP-1H alone or in combination with chemotherapy.