We studied the influence of the growth factor (GF) source, concentration and production time on the plating efficiency of Paracoccidioides brasiliensis yeast cells. The highest plating efficiencies were achieved when the GF was derived from a fast growing P. brasiliensis isolate which was not homologous to the plated samples.The colony forming units (c.f.u.) procedure is widely used when quantitation of viable micro-organisms is desired such as in studies concerning killing of bacteria or fungi by specific and non-specific mechanisms or the effect of therapeutic agents. The method entails plating the inoculum onto an appropriate semi-solid culture medium and counting the number of c.f.u, after an adequate incubation period. Plating efficiency is crucial in this method and has been defined as the number of colonies formed divided by hemacytometer counts of viable units, usually expressed as percentages [4].The application of the c.f.u, method to the dimorphic fungus Paracoccidioides brasiliensis, employing standard mycological media, presents a very low plating efficiency [4,7]. Improved efficiency was obtained by Castaneda et al.[2] using culture media (brain heart infusion agar (BHI), or modified McVeigh-Morton agar) supplemented with 5% culture filtrate from P. brasiliensis, as a source of growth-promoting factor (GF), and 4% horse serum.We have studied the effect of the GF source, concentration and time produced on the plating efficiency of P. brasiliensis samples in their yeast form. These studies were conducted by plating two P brasiliensis isolates, Pbl8AT, derived from a human isolate (Pbl8) which became attenuated by continuous in vitro subculture, and Pbl8R, which was reisolated from mice infected with Pbl8AT and is highly virulent [6].As GF sources we used broth filtrates from yeast cultures of Pbl8AT, Pbl8R and Pb192. Pb192 is a human isolate obtained from the fungal culture collection of the