Environmental influences on the new surfactant-stabilized bubbles, ST68, were investigated. We have developed a new surfactant-based contrast agent ST68, which is prepared by insonating buffered mixtures of Span 60 and Tween 80 in the presence of either air, PFC, or SF(6) gas. The effect of dilution, shear, and sonication on size distribution of ST68 showed that PFC-containing bubbles (ST68-PFC) were most stable. ST68-PFC bubbles lasted more than 15 min with approximately 30 dB backscatter enhancement in degassed phosphate-buffered saline, (pH 7.4), and air bubbles lasted approximately 3 s, suggesting the effects of diffusion. Additionally, it was found that the ionic strength of the suspending medium (for example, PBS), did not have any effect on ST68 bubbles containing SF(6) or PFC, but had a dramatic impact on bubbles containing air.