A two-piece plastic holder for loosely holding microliter sample tubes of a preselected design, each tube having a cylindrically shaped upper section open at its top end, a closed, tapered lower section extending downwardly therefrom, and a circumferential shoulder extending outwardly from the upper section at a position on the upper section spaced from the open end thereof. The holder includes a one-piece tray member and a one-piece retainer. The one-piece tray member including a flat, horizontal plate section containing holes in a rectangular array, a first vertical tray sidewall section around the plate extending upwardly to a height greater than the height of a tube resting in one of the holes, and a second vertical tray sidewall section around the plate extending downwardly approximately to the bottom of the upper section of a tube resting in one of the holes. The one-piece retainer is releasably engageable inside the tray over any sample tubes resting in the tray. The retainer comprises a flat, horizontal plate section containing holes in a rectangular array, and a vertical retainer sidewall section around the retainer plate section extending upwardly from the plate. When the retainer is engaged inside the tray, the retainer plate section lies slightly above the shoulder of a tube resting in the tray and the first tray sidewall section is about as high as the retainer sidewall section, so that tubes resting in the tray are retained loosely both vertically and laterally.