The effect of sampling time on the biological collection efficiency of viable airborne microorganisms was studied with two personal filter samplers (the Button Inhalable Aerosol Sampler and the 37-mm cassette) under relative humidities (RH) of 30 and 85%. Polycarbonate filters mounted in the samplers were challenged with five aerosolized microorganisms (fungal spores, endospores, and bacterial vegetative cells) and air was drawn through the samplers for a period of time ranging from 2 min to 8 h. The microorganisms were extracted from the filters at extraction efficiency of 96-98% by vortexing and ultrasonic agitation. The suspension of extracted microorganisms was analyzed by cultivation and by epifluorescence microscopic counting. Most of the tests did not indicate statistically significant differences in the bioefficiency of the two samplers. The culturability of Penicillium melinii and Aspergillus versicolor fungal spores at low and high RH values of 30 and 85% decreased during the first few minutes, but remained approximately the same for sampling times ranging from 30 min to 8 h. The relative culturability of Bacillus subtilis endospores decreased with sampling time, and was less than or equal to 17% at RH = 30% and less than or equal to 32% at RH = 85%. Pseudomonas fluorescens and Serratia marcescens vegetative cells were culturable only if sampled for 10 min or less. This concluded that the bioefficiency of filter samplers not only depends on the microbial species. but also on the sampling time and relative humidity.