28 patients (26 with breast cancer and 2 with colon cancer) received mitomycin, mitoxantrone and methotrexate (MMM). Half the patients had grade III-IV leukopenia and 29% had grade III-IV thrombocytopenia. The median time of recovery to WHO grade 0 was 62 and 128 days, respectively. Thrombocytopenia and leukopenia were more frequent and longer lasting after the three-drug part of the therapy, which suggests a critical role for mitomycin in this toxicity.