Summary.Platelet concentrates have been prepared in standard PVC (PL‐ 146) packs and polyolefin (PL‐732) packs. Comparison of pH and hypotonic stress of the platelet concentrates has shown that significantly improved values are obtained after storage of the concentrates at 22|MoC in PL‐732 packs for 5 days than after storage for 3 days in PL‐146 packs. There was no significant difference under conditions of the test between vertical and horizontal agitation of the concentrates. Since both pH and hypotonic stress recovery have shown a relationship to post‐transfusion recovery and viability, evidence is presented to support the 5‐day storage period of concentrates prepared and stored in P