Comparative studies of the aggregability in vitro and ex vivo, and of the surface/volume ratio of 111In- and 51Cr-labelled human platelets were carried out. The ADP-induced aggregation in vitro of 111In-platelets was superior to that of 51Cr-platelets, as was that of 51Cr-platelets labelled in plasma as compared to 51Cr-platelets labelled in buffer. These differences seemed to be reversed in vivo, as identical collagen-induced aggregation responses were observed ex vivo when comparisons were made between 111In- and 51Cr-platelets, and between labelled and unmanipulated platelets. Morphometric determination of the surface/volume ratios of the labelled platelets indicated a higher degree of platelet activation of 51Cr-platelets labelled in buffer as compared to those labelled in plasma. In this respect, no difference seemed to be present between 111In- and 51Cr-platelets. The results of the ex vivo aggregation studies were unaffected by the time spent by the platelets in the circulation within 24 h post-injection, by platelet isolation yield, and by the medium used in 51Cr-labelling. Our results indicate that it will be possible to conduct comparative studies of simultaneously induced aggregation ex vivo of different platelet populations labelled with 111In and 51Cr.