Platelet concentrates (PC) were stored for 7 days at 22°C in either CPDA-1 plasma (control) or one of three plasma-free media, namely a Tyrodes solution (BTST) fortified with citrate and with additional phosphate for buffering effect; Plasma-Lyte A solution (PCD) fortified with dextrose and citrate; and Ringers/CPD-50 (10:1) solution (RCPD) buffered with phosphate, all with a pH of 7.4. Parameters monitored included platelet count, pH, hypotonic shock response, aggregation response to single and paired agonists, and release by platelets of endogenous LDH, ATP and 5HT. BTST was shown to be a satisfactory plasma substitute over the entire storage period. PCD gave similar results over 4 days, although the decrease in pH was more marked. In all test systems RCPD was shown to be a poor plasma substitute.