Changes in proteins synthesised by the infective third-stage larvae (L3) of the filarial nematode Brugia pahangi were examined with respect to the temperature shift encountered by the parasite as it migrates from insect to mammal, and the presence of serum in the culture medium. While the synthesis of a number of polypeptides is regulated by the temperature shift of the L3 from 28 degrees C to 37 degrees C in vitro, there is no evidence that serum has any significant effect on protein synthesis. Two complexes of small acidic polypeptides (22-24 kDa and 18 kDa) are synthesised for a limited period only by L3 transferred to 37 degrees C. One component of each complex appears to be constitutively expressed at 28 degrees C, but its synthesis is up-regulated at 37 degrees C, while the remaining members of each complex are synthesised only at 37 degrees C. Subjection of L3 and post-infective (p.i.) L3 to heat shock (41 degrees C) also induces synthesis of both complexes, indicating that these heat-inducible polypeptides are related to the family of small heat shock proteins. The possible role of the heat shock-related proteins in this important environmental transition is considered.