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An analytical method of calculating the tensile strength of composites, with perfectly bonded and randomly oriented short fibres, was described recently in this journal by Zhu et al. ;. When comparing the calculations with experimental results for aluminum alloys, reinforced with Al2O3 fibres in a three-dimensionally random array, the conclusions were incorrect. The authors did not recognize that in this type of composite, the transverse fibres often delaminate. This paper describes an alternative and simpler method of calculating the tensile strength, which includes both perfect bonding and the effect of delamination. This method has been applied previously to two-dimensional random systems and is here extended to three-dimensional systems. The calculated values of strength are in excellent agreement with the experimental values quoted by Zhu et al. Further support is provided by comparisons with more extensive data for other metal matrix composites. General conclusions for a three-dimensional random system are that: (i) perfect bonding will only provide a modest increase of strength; and (ii) delamination of the transverse fibres will drastically reduce the strength and cannot be tolerated.