The Rockefeller Foundation, UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases and EEC (grant no. TSD-M-010-F(RS).Differentiation between a non-infective and an infective Leishrnania promastigote population was demonstrated. Promastigotes in the stationary phase (day 5) were found to be highly infective in vitro to BALB/c mouse peritoneal macrophages, compared with those of the logarithmic phase (day 3). The infective promastigotes showed surface antigenic determinants different from non-infective ones. Polyclonal anti-3 day and anti-5 day antibodies were bound specifically to the surface of corresponding promastigotes in both SRIA and IFAT; no strong cross-reactions were observed otherwise. Also, polyclonal anti-5 day but not anti-3 day antibodies recognized efficiently the antigenic molecules on the surface of late stage (day 7) sandfly promastigotes. This clearly indicates the appearance of new antigenic molecules on the surface of infective promastigote forms. Intracellular multiplication of Leishmania was significantly inhibited by anti-5 day antibodies compared with anti-3 day antibodies. The presence of new surface molecules on late stage promastigotes may contribute to Leishmania infectivity.