Actinomycin D is generally administered by serial low-dose injection over 5-10 days. Recent recognition of prolonged serum and tissue half-lives suggests that high-dose intermittent injecton should be equally effective and less toxic. An intermitten single dose schedule was selected for this phase II trial of actinomycin D in 23 patients with advanced breast cancer refractory to standard combination chemotherapy. The drug was given in doses of 0.75-1.5 mg/m2 at 2-week intervals or on days 1 and 8 of 28-day treatment cycles. One patient obtained a partial response with a duration of 5.7 months. Four patients experienced stabilization of advanced disease, with a mean duration of response of 6.4 months. Gastrointestinal toxicity occurred in 47% of patients and mild to moderate myelosuppression in 39%. We conclude that actinomycin D in this dosage and schedule has limited activity in advanced breast cancer. Higher doses might result in increased response rates but would be associated with greater toxicity.