The second virial coefficient A2 was determined for atactic poly(methyl methacrylate) (a-PMMA) over a wide range of the weight-average molecular weight M(w) from 5.90 X 10(2) to 1.58 X 10(6) in acetone at 25.0-degrees-C, for 5.90 X 10(2) less-than-or-equal-to M(w) less than or similar 2.0 X 10(5) in acetonitrile at 44.0-degrees-C (theta), and for M(w) greater than of similar 2.0 X 10(5) in chloroform at 25.0-degrees-C, in nitroethane at 30.0-degrees-C, and in acetonitrile at 47.0, 50.0, and 55.0-degrees-C. It is shown that the observed dependence of A2 on M(w) in the oligomer region may be quantitatively explained by the Yamakawa theory that takes into account the effect of chain ends. The values of the effective excess binary-cluster integrals beta1 and beta2 associated with the chain end beads are then found to be 62 and 910 angstrom3, respectively, in acetone at 25.0-degrees-C and -75 and 800 angstrom3, respectively, in acetonitrile at 0 by taking the repeat unit as a single bead. The analysis shows that the effect of chain ends remains even for relatively large M(w) (congruent-to 10(5)) both in the theta and good solvents as in the case of atactic polystyrene (a-PS). The results for the true interpenetration function psi in A2 without the effect of chain ends indicate that the two-parameter theory breaks down completely, as found previously for a-PS; the observed psi as a function of the cubed radius expansion factor alpha(S)3 depends separately on M(w) and on the excluded-volume strength B. It is then shown that the observed behavior of psi and also the remarkable difference in it between a-PMMA and a-PS may be rather satisfactorily explained by the Yamakawa theory that takes into account the effect of chain stiffness on the basis of the helical wormlike chain.