The schedule-dependent effect of bleomycin (BLM) on human squamous cell carcinoma (SCC) was investigated using a xenografted SCC line originating from the head and neck region. A total dose of 200 mg BLM/kg was administered according to one of three different schedules: one single i.p. injection; i.p. injection every four hours for seven days; and continuous s.c. administration for seven days via osmotic mini-pump. Both single dose treatment and iterated injections caused significant retardation of tumour growth, and continuous infusion had a more pronounced effect, almost completely retarding increase in tumour volume; these differences in effect in between the treatment schedules were not, however, statistically significant. The effect of continuous BLM infusion on cell cycle phase distribution and histopathological picture was also recorded. During the first three days of infusion, there was an accumulation of cells in Go/G1-phase paralleled by a depletion of cells in S-phase. This was followed by a normalisation during the rest of the infusion period, concomitant with a transient increase of the cells in G2-phase. Histopathologically, there were no changes during the first four days. A swelling of the cytoplasm cold then be seen; and after seven days, scattered groups of necrotic cells were observed which later formed necrotic foci. It is noteworthy that the major perturbances in cell cycle phase distribution preceded both retardation of tumour volume growth and histopathological changes.