Poly(A)-binding protein (PABP) is an RNA-binding protein that binds specifically to the poly(A) tail of messenger RNAs in eukaryotes. The PABP/poly(A) tail complex has been implicated as being important in promoting the efficient initiation of translation as well as in maintaining the integrity of the mRNA. PABP binds poly(A) cooperatively with a packing density of one PABP molecule per 25 adenosine residues. We have investigated the complexes formed between purified PABP and poly(A) RNA using atomic force microscopy (AFM). PABP alone was observed to be primarily in a monomer form with a height of 1.0 +/- 0.2 nm. Following binding to poly(A), PABP appeared to be present in variable size complexes that bound lengthwise along the RNA. This size of the PABP/poly(A) complex appeared to be maximal, suggesting that PARR binding to poly(A) may be self-limiting. Poly(A) RNA alone appeared to contain a knob-like structure that largely disappeared once PABP was bound. The use of AFM has therefore provided potential new insights into the complexes formed by this RNA-binding protein. (C) 1997 Academic Press.