The effect of dipyrone along with doxorubicin and mitoxantrone was studied alone and in combination on the 3H-thymidine (3H-TdR) incorporation in P388 leukaemia sensitive (P388/S) and resistant (P388/ADR) to doxorubicin. Dipyrone 10––4M demonstrated minimal inhibitory effect on DNA biosynthesis in both the sensitive and resistant cells. Doxorubicin and mitoxantrone at equimolar concentrations, indicated time and dose-dependent inhibition in 3H-TdR incorporation in the sensitive cells. The inhibition was more at the higher drug concentrations at 4 h drug exposure. Mitoxantrone showed cross-resistance in P388/ADR compared to P388/S. Both the drugs along with 10––4M dipyrone in the incubating medium revealed synergistic inhibitory activity in P388/S and P388/ADR. Observations indicate circumvention of doxorubicin and mitoxantrone resistance in P388/ADR by d