We have studied the adsorption of the Gourlay Acholeplasma virus MVL-1 to the host cell Acholeplasma laidlawii JA-1. Successful adsorption depends primarily on some unknown action of the serum factor in the medium, but evaluation of various physical parameters indicates clearly that given this factor the kinetics is pseudo first order (K = 3 × 10 −9 cm 3 /min) and the mechanism ionic. Chiefly important are the ionic strength of the cation and pH (optimal Na + = 0.08 M, pH = 6). The system seems indifferent to whether the cation is Na, K, NH 4 , Ca, or Mg. There is little effect of temperature over the range 0 to 42 C. The diffusion constant of the virus, calculated from its geometry or its maximum adsorption rate, is consistent with its reported size and shape.