A direct method for measuring mean linear intercepts (Lm) was tested. A semi-automatic image analysis system (Zeiss, Videoplan) and an overlay of parallel lines were employed in determining Lm in intravascularly perfused guinea pig lungs. Actual chord lengths were recorded and the data was immediately processed. Spurious intercepts and manual data manipulation were avoided. This direct method of measurement proved to be simple, rapid and very consistent across three trials for each animal. Analysis of variance revealed no significant differences between the three trials (p = 0.22).