Needleless connectors have been developed recently as a means of reducing transmission of AIDS and other blood-borne diseases by accidental needle sticks. However, the potential for hemolysis induced by fluid stresses within the connector remained to be determined. The influence of needleless connectors on hemolysis was evaluated in simulated clinical blood draws. Blood from five volunteers was drawn with vacuum tubes and syringes through 16, 18, and 22 gauge needles and PosiFlow (Becton Dickinson, Franklin Lakes, NJ) and Clave (ICU Medical, San Clemente, CA) needleless connectors. Hemolysis was measured in the samples by a spectrophotometric technique. Results showed that hemolysis increased when needleless connectors were used. The PosiFlow connector produced more hemolysis than the Clave device. Curiously, erythrocyte damage was greatest for connectors used with 18 gauge needles. Hemolysis was larger for samples drawn with vacuum tubes than with syringes. However, no combinations of connector, needle size, or blood draw device resulted in mean hemolysis values large enough to interfere with clinical assays.