A study of the strength of mortar-filled steel tubes was carried out by H. Salani and the Author during 1959-60. Long tubes under axial compression and under combined axial compression and bending were investigated by H. Salani, short specimens under axial compression and long tubes under lateral bending were considered by the Author. Preliminary investigations showed many more variables than had been considered by previous investigations in this field. Satisfactory consideration of these factors is shown to be essential before satisfactory strength characteristics may be determined and the investigation delineates these preliminary areas for study. Previous work calls attention to the unsatisfactory methods used in casting test columns and control samples. Although these discrepancies were known they were usually neglected but are considered in detail in this report. The plastic flow of concrete under load has also been noted previously but assumes very considerable importance in this present investigation. The effect of rate of load is investigated and finally the need for information on the plastic failure of axially loaded short steel tubes is pointed out. Previous work in this field is considerable and no attempt is made to evaluate this in. detail. However, summaries of some references are given in the appendix.