Background and Objectives  We previously developed a novel additive solution (M‐sol) with a high ability to preserve the in vitro qualities of platelets (PLTs) in washed PLTs Here, we compared the ability of M‐sol with that of commercially available additive solutions (ASs) to preserve the in vitro qualities (pH, mean PLT volume, %disc, P‐selectin, %hypotonic shock response and aggregation) of PLTs at a low plasma concentration. Materials and Methods  The platelet concentrate was divided into two equal aliquots (control group and test group). After centrifugation of both groups and removal of as much supernatant as possible, the pellet of the control group was resuspended in M‐sol and those of the test group were resuspended in other ASs, and subsequently stored in polyolefin bags with agitating at 20–24°C. Results  Compared with those stored in M‐sol, the qualities of PLTs stored in PAS‐B (alternative name; PAS‐II or T‐sol), PAS‐ C (alternative name; PAS‐III or Intersol) or Plasma Lyte were degraded as early as 24 h after washing. The qualities of PLTs stored in PAS‐D (alternative name; Composol PS) or PAS‐E (alternative name; PAS‐IIIM or SSP+) were comparable to that of those stored in M‐sol 24 h after washing; however, the qualities had deteriorated 72 h after washing. Conclusions  At a low plasma concentration (5% or less), the M‐sol showed a higher ability to preserve PLTs than the five ASs studied here. Although PAS‐D and PAS‐E are available as an AS for short‐term storage of washed PLTs, M‐sol is thought to be preferable for longer storage.