Intracytoplasmic A particles (CAP), previously identified as probably cytoplasmic nucleocapsid precursor structures to mouse mammary tumour virus (MMTV), possess both DNA binding and DNA unwinding activities, CAP proteins bind to both single-stranded (ss)- and double-stranded (ds)DNA, with the ssDNA slightly preferred. This activity was linear over a 30-fold concentration of A particle protein and was not affected by NaCl concentrations as high as 0.6 M or pH changes over a wide range. DNA binding by CAP proteins was sensitive to heat or addition of sodium dodecyl sulphate (SDS) and was neutralized by pre-incubation of CAP with anti-MMTV p14, but not by anti-MMTV p27, p10 or anti-mouse casein. Incubation of CAP with dsDNA led to unwinding of the double helix as measured by its increased sensitivity to S1 nuclease digestion. This activity was also linear over a several-fold concentration of A particle protein and was heat labile. It was not inhibited by pre-incubation of CAP with either anti-MMTV p14 or with anti-MMTV, anti-MMTV p27 and anti-MMTV p10. DNA unwinding was inhibited by anti-A particle antiserum and to a lesser extent by anti-CAP p20-18.